diff --git a/examples/online-boutique/services/cart/cart.go b/examples/online-boutique/services/cart/cart.go
index f13675a017..f73cd9aada 100644
--- a/examples/online-boutique/services/cart/cart.go
+++ b/examples/online-boutique/services/cart/cart.go
@@ -8,20 +8,20 @@ import (
 var store = NewStore()
 
 type Item struct {
-	ProductID string
-	Quantity  int
+	ProductID string `json:"productID"`
+	Quantity  int    `json:"quantity"`
 }
 
 type AddItemRequest struct {
-	UserID string
-	Item   Item
+	UserID string `json:"userID"`
+	Item   Item   `json:"item"`
 }
 
 type AddItemResponse struct{}
 
 type Cart struct {
-	UserID string
-	Items  []Item
+	UserID string `json:"userID"`
+	Items  []Item `json:"items"`
 }
 
 //ftl:verb
@@ -32,17 +32,17 @@ func AddItem(ctx context.Context, req AddItemRequest) (AddItemResponse, error) {
 }
 
 type GetCartRequest struct {
-	UserID string
+	UserID string `json:"userID"`
 }
 
 //ftl:verb
 //ftl:ingress GET /cart
 func GetCart(ctx context.Context, req GetCartRequest) (Cart, error) {
-	return Cart{Items: store.Get(req.UserID)}, nil
+	return Cart{Items: store.Get(req.UserID), UserID: req.UserID}, nil
 }
 
 type EmptyCartRequest struct {
-	UserID string
+	UserID string `json:"userID"`
 }
 
 type EmptyCartResponse struct{}
diff --git a/examples/online-boutique/services/cart/store.go b/examples/online-boutique/services/cart/store.go
index 9f36658bab..536ec527c0 100644
--- a/examples/online-boutique/services/cart/store.go
+++ b/examples/online-boutique/services/cart/store.go
@@ -19,20 +19,28 @@ func NewStore() *Store {
 	return &Store{carts: cache}
 }
 
-func (s *Store) Add(userID string, item Item) {
+func (s *Store) Add(userID string, newItem Item) {
 	s.lock.Lock()
 	defer s.lock.Unlock()
 	items, ok := s.carts.Get(userID)
-	if ok {
-		for i, item := range items {
-			if item.ProductID == item.ProductID {
-				items[i].Quantity += item.Quantity
-				break
-			}
+	if !ok {
+		s.carts.Add(userID, []Item{newItem})
+		return
+	}
+
+	found := false
+	for i, existingItem := range items {
+		if existingItem.ProductID == newItem.ProductID {
+			items[i].Quantity += newItem.Quantity
+			found = true
+			break
 		}
-	} else {
-		items = []Item{item}
 	}
+
+	if !found {
+		items = append(items, newItem)
+	}
+
 	s.carts.Add(userID, items)
 }