diff --git a/make_post_sell/tests/test_functional.py b/make_post_sell/tests/test_functional.py
index 3b6c132..f41da35 100644
--- a/make_post_sell/tests/test_functional.py
+++ b/make_post_sell/tests/test_functional.py
@@ -6623,10 +6623,9 @@ class TestAuctionCheckout(_AuthenticatedBase):
         self.testapp.get("/log-out")
         self.log_in_user(self.user2_creds)
 
-        # POST /a/{id}/checkout redirects to /cart.
-        self.testapp.post(f"/a/{auction_id}/checkout", status=302)
+        # POST /a/{id}/checkout redirects into the new cart by id.
+        res = self.testapp.post(f"/a/{auction_id}/checkout", status=302)
 
-        # The user's active cart now has the cart_auction association.
         from ..models.cart import Cart
         from ..models.cart_auction import MpsCartAuction
         from ..models.user import get_or_create_user_by_email
@@ -6636,13 +6635,13 @@ class TestAuctionCheckout(_AuthenticatedBase):
             .filter(Cart.user_id == winner.id, Cart.active.is_(True))
             .all()
         )
-        # At least one active cart with an MpsCartAuction row.
-        ca_count = self.dbsession.query(MpsCartAuction).count()
-        self.assertGreaterEqual(ca_count, 1)
-        # That cart's total uses the winning bid amount.
-        match = [c for c in active_carts if c.cart_auctions]
-        self.assertEqual(len(match), 1)
-        self.assertEqual(match[0].total_price_in_cents, 3700)
+        # Exactly one active cart, and it's the auction cart with the
+        # product in it — not a stale empty cart.
+        self.assertEqual(len(active_carts), 1)
+        self.assertTrue(active_carts[0].cart_auctions)
+        self.assertFalse(active_carts[0].is_empty)
+        self.assertEqual(active_carts[0].total_price_in_cents, 3700)
+        self.assertIn(f"/cart/{active_carts[0].uuid_str}", res.location)
 
     def test_non_winner_checkout_blocked(self):
         auction_id = self._ended_auction_with_winner()
@@ -6734,7 +6733,7 @@ class TestOfferCheckout(_AuthenticatedBase):
         self.testapp.get("/log-out")
         self.log_in_user(self.user2_creds)
 
-        self.testapp.post(f"/o/{offer_id}/checkout", status=302)
+        res = self.testapp.post(f"/o/{offer_id}/checkout", status=302)
 
         from ..models.cart import Cart
         from ..models.cart_offer import MpsCartOffer
@@ -6748,6 +6747,15 @@ class TestOfferCheckout(_AuthenticatedBase):
         match = [c for c in carts_with_offer if c.cart_offers]
         self.assertEqual(len(match), 1)
         self.assertEqual(match[0].total_price_in_cents, 7500)
+        # Regression: the offer cart must be the buyer's ONE active cart
+        # and must contain the product — otherwise /cart lands on a stale
+        # empty cart ("pressing Pay didn't add the item").
+        active_carts = [c for c in carts_with_offer if c.active]
+        self.assertEqual(len(active_carts), 1)
+        self.assertTrue(active_carts[0].cart_offers)
+        self.assertFalse(active_carts[0].is_empty)
+        # ...and the redirect lands on that cart by id.
+        self.assertIn(f"/cart/{match[0].uuid_str}", res.location)
 
     def test_non_buyer_checkout_blocked(self):
         offer_id = self._accepted_offer()
diff --git a/make_post_sell/views/auction.py b/make_post_sell/views/auction.py
index ae8c746..b553f18 100644
--- a/make_post_sell/views/auction.py
+++ b/make_post_sell/views/auction.py
@@ -298,7 +298,6 @@ def auction_checkout(request):
     Cart total is overridden to auction.winning_bid amount via the
     cart_auction association (Cart.auction_offer_override_in_cents).
     """
-    from ..models.cart import Cart
     from ..models.cart_auction import MpsCartAuction
 
     auction = get_auction_by_id(
@@ -319,16 +318,13 @@ def auction_checkout(request):
         request.session.flash(("Only the auction winner can check out.", "error"))
         return HTTPFound(f"/a/{auction.uuid_str}")
 
-    # Build a fresh cart for this auction. The product is added with
-    # quantity=auction.quantity so update_inventory() (cart success hook)
-    # naturally deducts the right number of units from inventory.
-    # Cart total comes from auction.winning_bid (override), not the
-    # quantity × product.price computation.
-    cart = Cart(user=request.user)
-    cart.shop = auction.shop
-    cart.active = True
-    request.dbsession.add(cart)
-    request.dbsession.flush()
+    # Build a fresh active cart for this auction (create_new_cart_for_user
+    # deactivates the user's other carts for this shop and activates this
+    # one — a hand-set cart.active = True left two active carts and /cart
+    # resolved to the wrong, empty one). The product is added with
+    # quantity=auction.quantity so update_inventory() deducts the right
+    # number of units; cart total comes from the cart_auction override.
+    cart = auction.shop.create_new_cart_for_user(request.user)
     cart.add_product(auction.product)
     if auction.quantity > 1:
         cart.set_product_quantity(auction.product, auction.quantity)
@@ -336,4 +332,4 @@ def auction_checkout(request):
         MpsCartAuction(cart=cart, auction=auction)
     )
     request.dbsession.flush()
-    return HTTPFound("/cart")
+    return HTTPFound(f"/cart/{cart.uuid_str}")
diff --git a/make_post_sell/views/offer.py b/make_post_sell/views/offer.py
index 65b7a7e..01502f2 100644
--- a/make_post_sell/views/offer.py
+++ b/make_post_sell/views/offer.py
@@ -341,11 +341,10 @@ def offer_withdraw(request):
 @view_config(route_name="offer_checkout", request_method="POST")
 @user_required(flash_msg="Please log in to pay.")
 def offer_checkout(request):
-    """Buyer checkout for an accepted offer: build a cart linked to the
-    offer, redirect to /cart. Cart total is overridden to the agreed
+    """Buyer checkout for an accepted offer: build a fresh cart linked to
+    the offer and redirect into it. Cart total is overridden to the agreed
     amount via the cart_offer association.
     """
-    from ..models.cart import Cart
     from ..models.cart_offer import MpsCartOffer
     from ..models.offer import OFFER_STATE_ACCEPTED
 
@@ -359,15 +358,17 @@ def offer_checkout(request):
         request.session.flash(("Only the buyer can check out.", "error"))
         return HTTPFound(f"/o/{offer.uuid_str}")
 
-    cart = Cart(user=request.user)
-    cart.shop = offer.shop
-    cart.active = True
-    request.dbsession.add(cart)
-    request.dbsession.flush()
+    # create_new_cart_for_user flips every other cart for this user+shop
+    # to inactive and activates the new one — so /cart resolves to it.
+    # (Setting cart.active = True by hand left two active carts and /cart
+    # would land on the old empty one.)
+    cart = offer.shop.create_new_cart_for_user(request.user)
     cart.add_product(offer.product)
     request.dbsession.add(MpsCartOffer(cart=cart, offer=offer))
     request.dbsession.flush()
-    return HTTPFound("/cart")
+    # Land directly in the cart by id — works regardless of which shop the
+    # request happens to be scoped to (SaaS domain vs the offer's shop).
+    return HTTPFound(f"/cart/{cart.uuid_str}")
 
 
 @view_config(route_name="shop_offers", renderer="shop_offers.j2")