How do I add PayPal 'donation' payment to my form/database?

Site visitors will come to my form in order to submit work. Here’s exactly what I want visitors to do:

  1. Visit the form
  2. Fill in the form and upload work.
  3. Enter the amount they want to pay
  4. Click Submit (Payments will go to my PayPal)
  5. End on a Thank You page.
  6. Their Entry ends up on my database and I can see who paid and how much.

It seems so simple but I’ve been struggling for days!

Submissions are Pay What You Want. So I guess that’s a donation… How do I allow paid submissions via PayPal on my custom database form (with a custom button)?
I’ve tried adding the various PayPal buttons/donation from the App Market, but they either allow a small number of transactions or they have no way of connecting specifically to the form (that I can figure out).

I’m not very code savvy, but is there a way to do this?