How to tie checkout for a digital good to a form submission together without 3party tools

@Gail_Ranger So custom code that intervenes where exactly? Can you elaborate a little? From your description of workflow C, I interpret 2 scenarios:

  1. a simple redirect to a checkout page after submitting a form?
  2. an integrated checkout at the end of a form, but preceding form submission AND checkout button?

I assume you are refering to scenario2 where custom code handles this workflow?