Purchase/Login Scripts?

[pxlcreations]

Hi, I need something that will let a user buy a web app from my site though paypal, be sent a specific username/password, can login or change password, and then is presented with the app. Is there anything like this?

[kaos]

What your asking is very complex... You need to give a little more information and, by the way, what code will this need to be written in?