PayPal integration help

Hello, I am currently constructing a website but I want a system where people can become members but membership will cost. I want to be able to use PayPal for people to purchase membership but I am unsure how to achieve the instant activation part once the person has paid. Thanks for the help in advance, James

I use the Paypal IPN option for that part of the processing.

I have it on my website, but it was a huge pain to do, took like 20 times to get it successfully working.

P.S. Look into API Callback