Track non registered users

Question

I had already gone through similar questions, but those does not answer my question.

I have a weird scenario. In my application (which is fundraising application) there are two types of users.

1. Registered users
2. Non registered users

Both types of users can pay with PayPal. I need to restrict both types of users so they can't pay more than FOUR time to the same fundraiser. Registered users can be restricted and its quite clear, but how I can restrict non registered users ? Thoughts ?

Answer 1

Use PayPal IPN services. IPN return all the required information which also consists email address, based on that email address you can keep track of non-registered users.