Stuck. Need urgent help!

Hi there,

We are building a website to help medical school applicants with their admission by providing help such as personal statement review services. We want people to upload their personal statement, then using a single button proceed to checkout and submit the form to the database after the payment has been completed. As the attached image shows, at the moment there are two buttons 1) submit the form 2) add to basket to make a payment.
We do not want to receive the form on the database before the payment has been received.
We tried to contact Wix support but they have told us that Wix Forms do not have a payment feature, and that we’ll have to use other premium app forms which only allow Paypal payments. which is very limited.

Can someone please give us some steps to take in order to make it all in 1 button and submit the form after the payment is completed?
Are there premium app forms that allow more than just paypal as a payment method?

We would really appreciate a clarification on this as we are really stuck!