Fix: load membership and customer with the correct helpers in PayPal Express IPN

inc/gateways/class-paypal-gateway.php

@@ -954,10 +954,11 @@
 
 		$custom = ! empty($posted['custom']) ? explode('|', (string) $posted['custom']) : [];
 
-		if (is_array($custom) && ! empty($custom)) {
+		// `custom` is built as "payment_id|membership_id|customer_id" (see set_express_checkout()).
+		if (is_array($custom) && count($custom) >= 3) {
 			$payment    = wu_get_payment(absint($custom[0]));
-			$membership = wu_get_payment(absint($custom[1]));
-			$customer   = wu_get_payment(absint($custom[2]));
+			$membership = wu_get_membership(absint($custom[1]));
+			$customer   = wu_get_customer(absint($custom[2]));
 		}
 
 		if ( ! empty($posted['recurring_payment_id'])) {
@@ -1387,17 +1388,17 @@
 				$membership->set_gateway_customer_id($details['PAYERID']);
 				$membership->set_gateway('paypal');
 
 			if (Payment_Status::COMPLETED === $payment_status) {
 				$membership->add_to_times_billed(1);
 
 				/*
 				 * Lets deal with upgrades, downgrades and addons
 				 *
 				 * Here, we just need to make sure we process
 				 * a membership swap.
 				 */
 				if ('upgrade' === $cart->get_cart_type() || 'addon' === $cart->get_cart_type()) {
 					$membership->swap($cart);
 
 					$membership->renew(true);
 				} elseif ('downgrade' === $cart->get_cart_type()) {