Attached is a ZIP file with everything you requested. I left the Auth.net stuff, as it won't let me leave it blank, but I made it gibberish.
Thanks for looking at all of this. To be honest, I have tried so many things now that I can't exactly explain exactly what my plan of attack was with these particular files.
All I need to do is have a coupon code that can be used three times and no more. I have a column in my visitors table called "coup_use" and it is to store how many times the coupon has been used, and if it is more than three, they can't use it again. I have tried starting with 3, 0, NULL, and increments of 50 (as the discount is $50).... Anything to make it work.
The form to submit the coupon code is on the cart.php page. The coupon itself is working fine, it's updating the column in the Visitors table that has me scratching my head.