Class Registration Plugin

Here’s what I’m trying to do.

My business is going to be offering basic classes to groups of 10 individuals. I’m looking for a way for attendees to sign up and pay online. But here’s the tricky part. A lot of attendees will have paid for their class through a promotion on an external website. Each customer will be given a unique code. What I would like to do is have a standard sign up for a class and pay, but with the ability to enter their unique id and thus bypass the payment process.

I’ve been tinkering with Events Espresso, but I can’t have it check the promo code against the master list of customers who have already paid.

Does anyone know of a plugin than could be fananlged to do this?