i am not entirely sure, also, this is more of a web-dev question than an ebook one
normally, you cannot "hide" urls.
a page links to something, that url gets displayed.
you could put the entire purchasing-process in a frame inside your website.
this sort of hides the urls, but it might cause problems when you mix secure and non-secure pages in one frameset.
i think you might have to work with paypal, they need to call a webpage on your server with a certain parameter, you then use this parameter and work with it.
ideally, start a session and continue on to another page, if it's fast then the user won't see the landing url on your page.
not knowing why you want to keep urls hidden, this answer is hard to answer.