As I said earlier, right now “modal” checkout is the only flow that Velo Wix Pay API supports.
If you want to have a checkout on a separate/embedded/collapsible page you need to look into dedicated solutions/Apps like Stores/Bookings/Events/Restaurants/etc.
In order to help you further with Wix Pay API we need to understand what exactly you think goes wrong during the checkout, ideally with screencast/screenshot.