In my knoweledge, the only way to make it secure, is that you have a script that will verify the payment before it allowes downloading the files. This can only be acheived by using a Shopping Cart capable of managing downloadable products. I believe that OScommerce does that, as well as Soholaunch
Among the possible solutions, my own IDC (Instant Download Cart) which allows to upload products, set your paypal and administrator details yourself, and, of course all the well known features (Add to Cart, view Cart, Continue Shopping, Checkout with PayPal, and pages customized to suit your site style.) If you are interested you can contact me.

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Navaldesign
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