Describe the bug
Encountered an issue with our Smart Coupons plugin affecting custom fields on the checkout page. When utilizing GPay/Apple Pay, the Checkout form data does not include information from these custom fields.

Upon inspecting the getOrderData function (ref), it appears that the function generates a checkout form data with predefined, hardcoded fields. Unfortunately, there is no mechanism to extend or add additional data, similar to WooCommerce checkout on the PHP side.

Also, incorporating additional data through JS code in the AJAX call is not feasible due to the use of iframes for rendering GPay/Apple Pay payment buttons.

Do let me know if any further details are needed for this.