pro sites - how can I spoof a paypal IPN so i can integrate this with other carts?

Hello,

I want to spoof a paypal IPN, however, I would like a little more understanding on how this works.

From what I gather, a transaction ID is generated the pro sites app on submit of checkout. Then paypal API passes it back.

I would like to use an outside cart process to allow for Coupons, Discounts, Etc.

Could someone hint at where to look for information on how to integrate other carts.

Or do you have any other suggestions? Its a great product, however, I need a method of being able to collect coupons, have setup fees on top of subscriptions, etc; and have the sites activated when they order (vs manual management and book keeping)