fix: shipping cost calculation and performance optimization

- Fix shipping cost not included in checkout total
- Add useShippingMethodSelector hook for proper abstraction
- Remove blocking initCheckout from Header for better performance
- Checkout now initializes lazily when cart opens or item added

src/app/[locale]/checkout/page.tsx

@@ -19,6 +19,7 @@ import { DEFAULT_PAYMENT_METHOD } from "@/lib/config/paymentMethods";
 import { GET_CHECKOUT_BY_ID } from "@/lib/saleor/queries/Checkout";
 import type { Checkout } from "@/types/saleor";
 import { createCheckoutService, type Address } from "@/lib/services/checkoutService";
+import { useShippingMethodSelector } from "@/lib/hooks/useShippingMethodSelector";
 
 interface ShippingAddressUpdateResponse {
   checkoutShippingAddressUpdate?: {
@@ -31,6 +32,8 @@ interface CheckoutQueryResponse {
   checkout?: Checkout;
 }
 
+
+
 interface ShippingMethod {
   id: string;
   name: string;
@@ -92,8 +95,16 @@ export default function CheckoutPage() {
   const [selectedShippingMethod, setSelectedShippingMethod] = useState<string>("");
   const [isLoadingShipping, setIsLoadingShipping] = useState(false);
 
+  // Hook to manage shipping method selection (both manual and auto)
+  const { selectShippingMethodWithApi } = useShippingMethodSelector({
+    checkoutId: checkout?.id ?? null,
+    onSelect: setSelectedShippingMethod,
+    onRefresh: refreshCheckout,
+  });
+
   const lines = getLines();
-  const total = getTotal();
+  // Use checkout.totalPrice directly for reactive updates when shipping method changes
+  const total = checkout?.totalPrice?.gross?.amount || getTotal();
 
   // Debounced shipping method fetching
   useEffect(() => {
@@ -150,7 +161,9 @@ export default function CheckoutPage() {
 
         // Auto-select first method if none selected
         if (availableMethods.length > 0 && !selectedShippingMethod) {
-          setSelectedShippingMethod(availableMethods[0].id);
+          const firstMethodId = availableMethods[0].id;
+          // Use the hook to both update UI and call API
+          await selectShippingMethodWithApi(firstMethodId);
         }
       } catch (err) {
         console.error("Error fetching shipping methods:", err);
@@ -210,6 +223,10 @@ export default function CheckoutPage() {
     setShippingAddress((prev) => ({ ...prev, email: value }));
   };
 
+  const handleShippingMethodSelect = async (methodId: string) => {
+    await selectShippingMethodWithApi(methodId);
+  };
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
 
@@ -576,7 +593,7 @@ export default function CheckoutPage() {
                                 name="shippingMethod"
                                 value={method.id}
                                 checked={selectedShippingMethod === method.id}
-                                onChange={(e) => setSelectedShippingMethod(e.target.value)}
+                                onChange={(e) => handleShippingMethodSelect(e.target.value)}
                                 className="w-4 h-4"
                               />
                               <span className="font-medium">{method.name}</span>

src/components/layout/Header.tsx

@@ -55,14 +55,14 @@ export default function Header({ locale: propLocale = "sr" }: HeaderProps) {
     setLangDropdownOpen(false);
   };
 
-  // Set language code first, then initialize checkout
+  // Set language code - checkout initializes lazily when cart is opened
   useEffect(() => {
     if (locale) {
       setLanguageCode(locale);
-      // Initialize checkout after language code is set
+      // Checkout will initialize lazily when user adds to cart or opens cart drawer
-      initCheckout();
+      // This prevents blocking page render with unnecessary API calls
     }
-  }, [locale, setLanguageCode, initCheckout]);
+  }, [locale, setLanguageCode]);
 
   useEffect(() => {
     const handleScroll = () => {

src/lib/hooks/useShippingMethodSelector.ts

@@ -0,0 +1,73 @@
+"use client";
+
+import { useCallback } from "react";
+import { createCheckoutService } from "@/lib/services/checkoutService";
+
+interface UseShippingMethodSelectorOptions {
+  checkoutId: string | null;
+  onSelect: (methodId: string) => void;
+  onRefresh: () => Promise<void>;
+}
+
+interface UseShippingMethodSelectorResult {
+  selectShippingMethod: (methodId: string) => Promise<void>;
+  selectShippingMethodWithApi: (methodId: string) => Promise<void>;
+}
+
+/**
+ * Hook to manage shipping method selection
+ * Encapsulates both UI state update and API communication
+ * Used for both manual selection (user click) and auto-selection (default method)
+ */
+export function useShippingMethodSelector(
+  options: UseShippingMethodSelectorOptions
+): UseShippingMethodSelectorResult {
+  const { checkoutId, onSelect, onRefresh } = options;
+
+  /**
+   * Updates UI state only (for initial/pre-selection)
+   */
+  const selectShippingMethod = useCallback(
+    async (methodId: string) => {
+      onSelect(methodId);
+    },
+    [onSelect]
+  );
+
+  /**
+   * Updates UI state AND calls Saleor API
+   * Use this when user manually selects OR when auto-selecting the default
+   */
+  const selectShippingMethodWithApi = useCallback(
+    async (methodId: string) => {
+      if (!checkoutId) {
+        console.warn("[selectShippingMethodWithApi] No checkoutId provided");
+        return;
+      }
+
+      // Update UI immediately for responsiveness
+      onSelect(methodId);
+
+      // Call API through CheckoutService
+      const checkoutService = createCheckoutService(checkoutId);
+      const result = await checkoutService.updateShippingMethod(methodId);
+
+      if (result.success) {
+        // Refresh checkout to get updated totals including shipping
+        await onRefresh();
+      } else {
+        console.error(
+          "[selectShippingMethodWithApi] Failed to update shipping method:",
+          result.error
+        );
+        // Could add error handling/rollback here
+      }
+    },
+    [checkoutId, onSelect, onRefresh]
+  );
+
+  return {
+    selectShippingMethod,
+    selectShippingMethodWithApi,
+  };
+}