diff --git a/src/pretix/base/services/orders.py b/src/pretix/base/services/orders.py
index ad940c99c7..8fdfa7173e 100644
--- a/src/pretix/base/services/orders.py
+++ b/src/pretix/base/services/orders.py
@@ -85,6 +85,9 @@ def mark_order_paid(order: Order, provider: str=None, info: str=None, date: date
     :type mail_text: str
     :raises Quota.QuotaExceededException: if the quota is exceeded and ``force`` is ``False``
     """
+    if order.status == Order.STATUS_PAID:
+        return order
+
     with order.event.lock() as now_dt:
         can_be_paid = order._can_be_paid()
         if not force and can_be_paid is not True:
diff --git a/src/pretix/plugins/paypal/payment.py b/src/pretix/plugins/paypal/payment.py
index fd441f8206..295905bb30 100644
--- a/src/pretix/plugins/paypal/payment.py
+++ b/src/pretix/plugins/paypal/payment.py
@@ -8,7 +8,7 @@ from django.contrib import messages
 from django.template.loader import get_template
 from django.utils.translation import ugettext as __, ugettext_lazy as _
 
-from pretix.base.models import Quota, RequiredAction
+from pretix.base.models import Order, Quota, RequiredAction
 from pretix.base.payment import BasePaymentProvider
 from pretix.base.services.mail import SendMailException
 from pretix.base.services.orders import mark_order_paid, mark_order_refunded
@@ -199,6 +199,10 @@ class Paypal(BasePaymentProvider):
             logger.error('Invalid state: %s' % str(payment))
             return
 
+        if order.status == Order.STATUS_PAID:
+            logger.warning('PayPal success event even though order is already marked as paid')
+            return
+
         try:
             mark_order_paid(order, 'paypal', json.dumps(payment.to_dict()))
         except Quota.QuotaExceededException as e:
diff --git a/src/pretix/presale/views/cart.py b/src/pretix/presale/views/cart.py
index 37c5dd8345..400a00cd0c 100644
--- a/src/pretix/presale/views/cart.py
+++ b/src/pretix/presale/views/cart.py
@@ -42,8 +42,10 @@ class CartActionMixin:
             amount = int(value)
         except ValueError:
             raise CartError(_('Please enter numbers only.'))
-        if amount <= 0:
+        if amount < 0:
             raise CartError(_('Please enter positive numbers only.'))
+        elif amount == 0:
+            return
 
         price = self.request.POST.get('price_' + "_".join(parts[1:]), "")
         if key.startswith('item_'):