What should be the impact of thread and sessions over paypal mechanism in asp.net c#

Now a days i am looking for the perfect time estimation for the asp.net 4.0 with c# integration. since workflow is while user subscribing on the site, he should pay for subscription. Well this is the normal usecase now a days. but how developers manages the sessions and threading while performing the payment transactions.

In fact i don not have much knowledge about threading and session and suddenly i have the task to bu开发者_Go百科ild this mechanism. well, how to manage all this ? so that transaction is more secure from our side. (sure paypal has taken care. and how many ways we can integrate paypal with asp.net application ?)

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You need to make a "script" that handles a url from paypal.

Ex. When user payment is confirmed paypal sends info on www.yousite.com/paypalhandler.aspx

This info contains userId etc etc. And now the script makes the necessary updates to your db.