I have a question regarding Paypal Buy Now Buttons.

I want my buyers to get immediate download page after they pay.

As paypal gives two options for creating Buy now Buttons - one with
encrypted form other using advanced option (unencrypted) which allows
you to add some extra info as well as url for thank you pages.

using second option does have vulnerability as thank You url (download
page) is exposed in Source code.Any one who gets access to source code
is able to see the complete url of thank u page (in my case the Download Page).

How to we eliminate this vulnerability ?

Is there any kind of encryption or some other way to do it ? or any other FREE online payment Processing which would accomplish this task without any hassles?

Actually i'm not well versed with paypal functionality.Can any body help me out to come with robust and secure solution.