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Personally, I think the advatage of this particular solution is that is is pure and simple HTML (ok, a tad of JS). For simple needs, it lets one rely on PayPal for the heavy lifting. For more, go with osCommerce.
So just take the sample form & script and embed it in your favorite module, block, template, etc... The possibilities are endless. Want to tie it to the XOOPS user profile? No sweat, just drop in the XOOPS variable for the user-email and pass it as a hidden input to the form.