Store API: Ensure Store API recalculates cart totals prior to running validation hooks (#49455)

* Recalc totals on checkout route before validation

* changelog

plugins/woocommerce/changelog/fix-calc-totals-before-running-validation

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Ensure the Store API recalculates cart totals prior to running validation hooks.

plugins/woocommerce/src/StoreApi/Routes/V1/Checkout.php

@@ -188,6 +188,12 @@ class Checkout extends AbstractCartRoute {
 	 * @return \WP_REST_Response
 	 */
 	protected function get_route_post_response( \WP_REST_Request $request ) {
+		/**
+		 * Before triggering validation, ensure totals are current and in turn, things such as shipping costs are present.
+		 * This is so plugins that validate other cart data (e.g. conditional shipping and payments) can access this data.
+		 */
+		$this->cart_controller->calculate_totals();
+
 		/**
 		 * Validate items etc are allowed in the order before the order is processed. This will fix violations and tell
 		 * the customer.