diff --git a/src/pretix/base/models.py b/src/pretix/base/models.py
index 9a6b0510bb..eeb6491076 100644
--- a/src/pretix/base/models.py
+++ b/src/pretix/base/models.py
@@ -1359,6 +1359,9 @@ class Quota(Versionable):
     class LockTimeoutException(Exception):
         pass
 
+    class QuotaExceededException(Exception):
+        pass
+
     def lock(self):
         """
         Issue a lock on this quota so nobody can take tickets from this quota until
@@ -1565,7 +1568,63 @@ class Order(Versionable):
         order.save()
         return order
 
-    def mark_paid(self, provider=None, info=None, date=None, manual=None):
+    def _can_be_paid(self):
+        error_messages = {
+            'unavailable': _('Some of the ordered products were no longer available.'),
+            'busy': _('We were not able to process the request completely as the '
+                      'server was too busy. Please try again.'),
+            'late': _("The payment is too late to be accepted."),
+        }
+
+        if self.event.settings.get('payment_term_last') \
+                and now() > self.event.settings.get('payment_term_last'):
+            return error_messages['late']
+        if now() < self.expires:
+            return True
+        if not self.event.settings.get('payment_term_accept_late'):
+            return error_messages['late']
+
+        positions = list(self.positions.all().select_related(
+            'item', 'variation'
+        ).prefetch_related(
+            'variation__values', 'variation__values__prop',
+            'item__questions', 'answers'
+        ))
+        quotas_locked = set()
+
+        try:
+            for i, op in enumerate(positions):
+                quotas = list(op.item.quotas.all()) if op.variation is None else list(op.variation.quotas.all())
+                if len(quotas) == 0:
+                    raise Quota.QuotaExceededException(error_messages['unavailable'])
+
+                for quota in quotas:
+                    # Lock the quota, so no other thread is allowed to perform sales covered by this
+                    # quota while we're doing so.
+                    if quota not in quotas_locked:
+                        quota.lock()
+                        quotas_locked.add(quota)
+                        quota.cached_availability = quota.availability()[1]
+                    else:
+                        # Use cached version
+                        quota = [q for q in quotas_locked if q.pk == quota.pk][0]
+                    quota.cached_availability -= 1
+                    if quota.cached_availability < 0:
+                        # This quota is sold out/currently unavailable, so do not sell this at all
+                        raise Quota.QuotaExceededException(error_messages['unavailable'])
+        except Quota.QuotaExceededException as e:
+            return str(e)
+        except Quota.LockTimeoutException:
+            # Is raised when there are too many threads asking for quota locks and we were
+            # unaible to get one
+            return error_messages['busy']
+        finally:
+            # Release the locks. This is important ;)
+            for quota in quotas_locked:
+                quota.release()
+        return True
+
+    def mark_paid(self, provider=None, info=None, date=None, manual=None, force=False):
         """
         Mark this order as paid. This clones the order object, sets the payment provider,
         info and date and returns the cloned order object.
@@ -1577,7 +1636,14 @@ class Order(Versionable):
         :param date: The date the payment was received (if you pass ``None``, the current
                      time will be used).
         :type date: datetime
+        :param force: Whether this payment should be marked as paid even if no remaining
+                      quota is available (default: ``False``).
+        :type force: boolean
+        :raises Quota.QuotaExceededException: if the quota is exceeded and ``force`` is ``False``
         """
+        can_be_paid = self._can_be_paid()
+        if not force and can_be_paid is not True:
+            raise Quota.QuotaExceededException(can_be_paid)
         order = self.clone()
         order.payment_provider = provider or order.payment_provider
         order.payment_info = info or order.payment_info
diff --git a/src/pretix/base/payment.py b/src/pretix/base/payment.py
index 1234fd9504..e6dadc0860 100644
--- a/src/pretix/base/payment.py
+++ b/src/pretix/base/payment.py
@@ -239,8 +239,11 @@ class BasePaymentProvider:
         If the payment is completed, you should call ``order.mark_paid(provider, info)``
         with ``provider`` being your :py:attr:`identifier` and ``info`` being any string
         you might want to store for later usage. Please note, that if you want to store
-        something inside ``order.payment_info``, please do a ``order = order.clone()`` before
-        modifying or saving the order object.
+        something inside ``order.payment_info``, please do it after the ``mark_paid`` call,
+        as this call does a object clone for you. Please also note that ``mark_paid`` might
+        raise a ``Quota.QuotaExceededException`` if (and only if) the payment term of this
+        order is over and some of the items are sold out. You should use the exception message
+        to display a meaningful error to the user.
 
         The default implementation just returns ``None`` and therefore leaves the
         order unpaid. The user will be redirected to the order's detail page by default.
diff --git a/src/pretix/base/settings.py b/src/pretix/base/settings.py
index ab8c9d667b..92e5ed967a 100644
--- a/src/pretix/base/settings.py
+++ b/src/pretix/base/settings.py
@@ -37,6 +37,10 @@ DEFAULTS = {
         'default': None,
         'type': datetime,
     },
+    'payment_term_accept_late': {
+        'default': 'True',
+        'type': bool
+    },
     'timezone': {
         'default': settings.TIME_ZONE,
         'type': str
diff --git a/src/pretix/control/templates/pretixcontrol/event/settings.html b/src/pretix/control/templates/pretixcontrol/event/settings.html
index 92de3fcf71..30f07b1c53 100644
--- a/src/pretix/control/templates/pretixcontrol/event/settings.html
+++ b/src/pretix/control/templates/pretixcontrol/event/settings.html
@@ -31,6 +31,7 @@
             {% bootstrap_field form.presale_end layout="horizontal" %}
             {% bootstrap_field sform.payment_term_days layout="horizontal" %}
             {% bootstrap_field sform.payment_term_last layout="horizontal" %}
+            {% bootstrap_field sform.payment_term_accept_late layout="horizontal" %}
             {% bootstrap_field sform.last_order_modification_date layout="horizontal" %}
         </fieldset>
         <fieldset>
diff --git a/src/pretix/control/views/event.py b/src/pretix/control/views/event.py
index 7a6a385938..e672e21d8f 100644
--- a/src/pretix/control/views/event.py
+++ b/src/pretix/control/views/event.py
@@ -61,6 +61,14 @@ class EventSettingsForm(SettingsForm):
                     "days configured above."),
         required=False
     )
+    payment_term_accept_late = forms.BooleanField(
+        label='Accept late payments',
+        help_text=_("Accept payments that come after the end of the order's payment term. "
+                    "Payments will only be accepted if the regarding quotas have remaining "
+                    "capacity. No payments will be accepted after the 'Last date of payments' "
+                    "configured above."),
+        required=False
+    )
     last_order_modification_date = forms.DateTimeField(
         label='Last date of modifications',
         help_text=_("The last date users can modify details of their orders, such as attendee names or "
diff --git a/src/pretix/control/views/orders.py b/src/pretix/control/views/orders.py
index 2eb69d7d68..7dd0a550da 100644
--- a/src/pretix/control/views/orders.py
+++ b/src/pretix/control/views/orders.py
@@ -8,7 +8,7 @@ from django.shortcuts import redirect, render
 from django.utils.functional import cached_property
 from django.views.generic import ListView, DetailView
 
-from pretix.base.models import Order
+from pretix.base.models import Order, Quota
 from pretix.base.signals import register_payment_providers
 from pretix.control.permissions import EventPermissionRequiredMixin
 
@@ -105,8 +105,12 @@ class OrderTransition(EventPermissionRequiredMixin, OrderView):
     def post(self, *args, **kwargs):
         to = self.request.POST.get('status', '')
         if self.order.status == 'n' and to == 'p':
-            self.order.mark_paid(manual=True)
-            messages.success(self.request, _('The order has been marked as paid.'))
+            try:
+                self.order.mark_paid(manual=True)
+            except Quota.QuotaExceededException as e:
+                messages.error(self.request, str(e))
+            else:
+                messages.success(self.request, _('The order has been marked as paid.'))
         elif self.order.status == 'n' and to == 'c':
             order = self.order.clone()
             order.status = Order.STATUS_CANCELLED
diff --git a/src/pretix/plugins/banktransfer/views.py b/src/pretix/plugins/banktransfer/views.py
index a86a7b313a..b9bb2c475d 100644
--- a/src/pretix/plugins/banktransfer/views.py
+++ b/src/pretix/plugins/banktransfer/views.py
@@ -8,7 +8,7 @@ from django.core.urlresolvers import reverse
 from django.shortcuts import redirect, render
 from django.utils.timezone import now
 from django.views.generic import TemplateView
-from pretix.base.models import Order
+from pretix.base.models import Order, Quota
 from pretix.control.permissions import EventPermissionRequiredMixin
 from pretix.plugins.banktransfer import csvimport, mt940import
 from django.utils.translation import ugettext_lazy as _
@@ -33,15 +33,25 @@ class ImportView(EventPermissionRequiredMixin, TemplateView):
         if 'confirm' in self.request.POST:
             orders = Order.objects.filter(event=self.request.event,
                                           code__in=self.request.POST.getlist('mark_paid'))
+            some_failed = False
             for order in orders:
-                order.mark_paid(provider='banktransfer', info=json.dumps({
-                    'reference': self.request.POST.get('reference_%s' % order.code),
-                    'date': self.request.POST.get('date_%s' % order.code),
-                    'payer': self.request.POST.get('payer_%s' % order.code),
-                    'import': now().isoformat(),
-                }))
+                try:
+                    order.mark_paid(provider='banktransfer', info=json.dumps({
+                        'reference': self.request.POST.get('reference_%s' % order.code),
+                        'date': self.request.POST.get('date_%s' % order.code),
+                        'payer': self.request.POST.get('payer_%s' % order.code),
+                        'import': now().isoformat(),
+                    }))
+                except Quota.QuotaExceededException:
+                    some_failed = True
 
-            messages.success(self.request, _('The selected orders have been marked as paid.'))
+            if some_failed:
+                messages.success(self.request, _('The selected orders have been marked as paid.'))
+            else:
+                messages.warning(self.request, _('Not all of the selected orders could be marked as '
+                                                 'paid as some of them have expired and the selected '
+                                                 'items are sold out.'))
+                # TODO: Display a list of them!
             return self.redirect_back()
 
         messages.error(self.request, _('We were unable to detect the file type of this import. Please '
diff --git a/src/pretix/plugins/paypal/payment.py b/src/pretix/plugins/paypal/payment.py
index 3a639aeaa6..686a960266 100644
--- a/src/pretix/plugins/paypal/payment.py
+++ b/src/pretix/plugins/paypal/payment.py
@@ -9,6 +9,7 @@ from django.utils.translation import ugettext as __
 from django import forms
 
 import paypalrestsdk
+from pretix.base.models import Quota
 
 from pretix.base.payment import BasePaymentProvider
 
@@ -175,8 +176,11 @@ class Paypal(BasePaymentProvider):
             logger.error('Invalid state: %s' % str(payment))
             return
 
-        order.mark_paid('paypal', json.dumps(payment.to_dict()))
-        messages.success(request, _('We successfully received your payment. Thank you!'))
+        try:
+            order.mark_paid('paypal', json.dumps(payment.to_dict()))
+            messages.success(request, _('We successfully received your payment. Thank you!'))
+        except Quota.QuotaExceededException as e:
+            messages.error(request, str(e))
         return None
 
     def order_pending_render(self, request, order) -> str:
diff --git a/src/pretix/plugins/stripe/payment.py b/src/pretix/plugins/stripe/payment.py
index 2f3a62b2fa..97739d42b5 100644
--- a/src/pretix/plugins/stripe/payment.py
+++ b/src/pretix/plugins/stripe/payment.py
@@ -5,6 +5,7 @@ from django.contrib import messages
 from django.template.loader import get_template
 from django.utils.translation import ugettext_lazy as _
 from django import forms
+from pretix.base.models import Quota
 import stripe
 
 from pretix.base.payment import BasePaymentProvider
@@ -67,7 +68,11 @@ class Stripe(BasePaymentProvider):
         )
         logging.info(charge)
         if charge.status == 'succeeded' and charge.paid:
-            order.mark_paid('stripe', str(charge))
+            try:
+                order.mark_paid('paypal', str(charge))
+                messages.success(request, _('We successfully received your payment. Thank you!'))
+            except Quota.QuotaExceededException as e:
+                messages.error(request, str(e))
             messages.success(request, _('We successfully received your payment. Thank you!'))
         else:
             messages.warning(request, _('Stripe reported an error: %s' % charge.failure_message))
diff --git a/src/tests/base/test_models.py b/src/tests/base/test_models.py
index e1445ac130..d348b7eaa0 100644
--- a/src/tests/base/test_models.py
+++ b/src/tests/base/test_models.py
@@ -176,7 +176,8 @@ class UserTestCase(TestCase):
         self.assertEqual(u.identifier, "test@example.com")
 
 
-class QuotaTestCase(TestCase):
+class BaseQuotaTestCase(TestCase):
+
     @classmethod
     def setUpTestData(cls):
         o = Organizer.objects.create(name='Dummy', slug='dummy')
@@ -187,7 +188,7 @@ class QuotaTestCase(TestCase):
 
     def setUp(self):
         self.quota = Quota.objects.create(name="Test", size=2, event=self.event)
-        self.item1 = Item.objects.create(event=self.event, name="Ticket")
+        self.item1 = Item.objects.create(event=self.event, name="Ticket", default_price=23)
         self.item2 = Item.objects.create(event=self.event, name="T-Shirt")
         p = Property.objects.create(event=self.event, name='Size')
         pv1 = PropertyValue.objects.create(prop=p, value='S')
@@ -197,6 +198,9 @@ class QuotaTestCase(TestCase):
         self.var1.values.add(pv1)
         self.item2.properties.add(p)
 
+
+class QuotaTestCase(BaseQuotaTestCase):
+
     def test_available(self):
         self.quota.items.add(self.item1)
         self.assertEqual(self.item1.check_quotas(), (Quota.AVAILABILITY_OK, 2))
@@ -289,3 +293,69 @@ class QuotaTestCase(TestCase):
         quota2.size = 0
         quota2.save()
         self.assertEqual(self.item1.check_quotas(), (Quota.AVAILABILITY_GONE, 0))
+
+
+class OrderTestCase(BaseQuotaTestCase):
+
+    def setUp(self):
+        super().setUp()
+        self.user = User.objects.create_local_user(self.event, 'dummy', 'dummy')
+        self.order = Order.objects.create(
+            status=Order.STATUS_PENDING, event=self.event,
+            user=self.user, datetime=now() - timedelta(days=5),
+            expires=now() + timedelta(days=5), total=46
+        )
+        self.quota.items.add(self.item1)
+        OrderPosition.objects.create(order=self.order, item=self.item1,
+                                     variation=None, price=23)
+        OrderPosition.objects.create(order=self.order, item=self.item1,
+                                     variation=None, price=23)
+
+    def test_paid_in_time(self):
+        self.quota.size = 0
+        self.quota.save()
+        self.order.mark_paid()
+        self.order = Order.objects.current.get(identity=self.order.identity)
+        self.assertEqual(self.order.status, Order.STATUS_PAID)
+
+    def test_paid_expired_available(self):
+        self.order.expires = now() - timedelta(days=2)
+        self.order.save()
+        self.order.mark_paid()
+        self.order = Order.objects.current.get(identity=self.order.identity)
+        self.assertEqual(self.order.status, Order.STATUS_PAID)
+
+    def test_paid_expired_partial(self):
+        self.order.expires = now() - timedelta(days=2)
+        self.order.save()
+        self.quota.size = 1
+        self.quota.save()
+        try:
+            self.order.mark_paid()
+            self.assertFalse(True, 'This should have raised an exception.')
+        except Quota.QuotaExceededException:
+            pass
+        self.order = Order.objects.current.get(identity=self.order.identity)
+        self.assertIn(self.order.status, (Order.STATUS_PENDING, Order.STATUS_EXPIRED))
+
+    def test_paid_expired_unavailable(self):
+        self.order.expires = now() - timedelta(days=2)
+        self.order.save()
+        self.quota.size = 0
+        self.quota.save()
+        try:
+            self.order.mark_paid()
+            self.assertFalse(True, 'This should have raised an exception.')
+        except Quota.QuotaExceededException:
+            pass
+        self.order = Order.objects.current.get(identity=self.order.identity)
+        self.assertIn(self.order.status, (Order.STATUS_PENDING, Order.STATUS_EXPIRED))
+
+    def test_paid_expired_unavailable_force(self):
+        self.order.expires = now() - timedelta(days=2)
+        self.order.save()
+        self.quota.size = 0
+        self.quota.save()
+        self.order.mark_paid(force=True)
+        self.order = Order.objects.current.get(identity=self.order.identity)
+        self.assertEqual(self.order.status, Order.STATUS_PAID)