Question : Simple PayPal Shopping Cart

I have a requirement that I believe cannot be met by the standard PayPal shopping cart, and I'm looking for recommendations about how to solve the issue.  

I need to enforce a minimum quantity limitation, so when you hit the "add to cart" step, you can add no less than 20 items, but as many as you want, so long as the quantity is at least 20.

This is easy enough to do in Javascript, but you lose control of the quantity when the client goes to PayPal to pay.  I'm looking for the simplest answer, since I have no need for other shopping cart features, just this minimum order limitation.  I'd also like to have the answer in PHP, since I'm fairly fluent in PHP and the rest of the site is PHP-driven.

Thanks and best regards to all, ~Ray

Answer : Simple PayPal Shopping Cart

Ray:

First of all Merry Christmas.  I've spotted many of your posts in ones that I've also answered.  Your support for the advice given by the other experts is appreciated.

I attempted a PayPal interface in a site I was doing for a client.  PayPal suggested their Direct Payment process, where PayPal processes the transaction in the background.  The user never leaves your site. I've attached the PDF version of the manual.  The overview of their Direct Payment process in on page 13.

I had a lot of issues getting it to work.  I achieved what I needed setting up a standard credit card processing interface through Authorize.net.  Took less than 2 hours for the basic interface, and it does what you need.  Any reason you're sticking with PayPal?

PayPal-PaymentsProPayflow-DevGui.pdf (1.3 MB) (File Type Details) 

PayPal PaymentProFlow Developers Guide