You MUST have PHP/MySQL & ability to work with existing code
I prepaired html/javascript/css for the screen, which you can see here:
[[login to view URL]][1]
Your goal is to:
#1. incorporate it into the Zen Cart item page.
#2. change the Checkout screen. Moc-Up: [[login to view URL]][1]
All or most of queries & functions exist in different files of Zen Cart, but you might need to write a few or correct existing ones.