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I don't have any experience with paypal or anything alike, but from what I read in other posts regarding this subject the answer seems to be this one:
you have to check the referrer url to see if the user is comming from the right page from the paypal site (btw, you have to know that page's url). If the urls are the same, then the user has payed for what you're selling so you can let him download the product.
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