diff --git a/pkg/cart/mutation_add_item.go b/pkg/cart/mutation_add_item.go
index 0e188e3..1269d58 100644
--- a/pkg/cart/mutation_add_item.go
+++ b/pkg/cart/mutation_add_item.go
@@ -42,7 +42,7 @@ func AddItem(g *CartGrain, m *messages.AddItem) error {
 			t := m.ReservationEndTime.AsTime()
 			if existing.ReservationEndTime == nil || existing.ReservationEndTime.Before(m.ReservationEndTime.AsTime()) {
 				existing.ReservationEndTime = &t
-				existing.Quantity = int(m.Quantity)
+				existing.Quantity += int(m.Quantity)
 			} else {
 				existing.ReservationEndTime = &t
 			}