fix: offer/auction cart charges agreed price, not list price

total_discounted_price_in_cents summed line items and ignored the
cart_offer / cart_auction override. is_discounted then reported True
(agreed ≠ list), so total_in_cents — what the charge path reads —
returned the list price. UI strikethroughed the agreed price and
charged the list price.

Short-circuit the discounted path on override, the same way
total_price_in_cents does. An offer/auction is a negotiated price,
not a discount, so coupons and gift-card balances do not stack on
top of it.

Extended both override tests to assert total_in_cents,
total_discounted_price_in_cents, and is_discounted — the gap that
let this ship.

make_post_sell/models/cart.py

@@ -395,7 +395,18 @@ class Cart(RBase, Base):
     def total_discounted_price_in_cents(self):
         """
         Calculate the total discounted price in cents, including handling cost and gift card purchases.
+        MPS-20 + MPS-21: an auction-won or offer-accepted cart pays the
+        agreed amount; coupons and gift-card balances do not stack on top
+        of a negotiated price.
         """
+        override = self.auction_offer_override_in_cents
+        if override is not None:
+            total = override
+            total += self.gift_card_purchases_total_in_cents
+            if self.handling_cost_in_cents:
+                total += self.handling_cost_in_cents
+            return total
+
         total = sum(self.discounted_shop_totals_in_cents.values())
         total += self.gift_card_purchases_total_in_cents
         if self.handling_cost_in_cents:

make_post_sell/tests/test_integration.py

@@ -4942,6 +4942,12 @@ class TestCartTotalOverride(DatabaseIntegrationTests):
         # Override: $42 not $100.
         self.assertEqual(cart.auction_offer_override_in_cents, 4200)
         self.assertEqual(cart.total_price_in_cents, 4200)
+        # The charge path (total_in_cents) and the discounted path must
+        # both honor the override — otherwise the cart strikethroughs the
+        # agreed price and charges the list price.
+        self.assertEqual(cart.total_discounted_price_in_cents, 4200)
+        self.assertEqual(cart.total_in_cents, 4200)
+        self.assertFalse(cart.is_discounted)
 
     def test_offer_override_uses_current_amount(self):
         from ..models.cart import Cart
@@ -4974,6 +4980,13 @@ class TestCartTotalOverride(DatabaseIntegrationTests):
         # Override: $80 not $100.
         self.assertEqual(cart.auction_offer_override_in_cents, 8000)
         self.assertEqual(cart.total_price_in_cents, 8000)
+        # Cart logic was built for coupons (a discount on top of list
+        # price). Offers are a negotiated price, not a discount — both
+        # totals must agree, so is_discounted is False and the buyer is
+        # charged the offer amount, not the list price.
+        self.assertEqual(cart.total_discounted_price_in_cents, 8000)
+        self.assertEqual(cart.total_in_cents, 8000)
+        self.assertFalse(cart.is_discounted)
 
 
 class TestAuctionTickIntegration(DatabaseIntegrationTests):