The most secure method of providing downloads for your product files would involve letting us handle the download-delivery fulfillment. Our system only issues download links to the buyer after the payment processor (PayPal/Google/etc.) confirms the buyer's payment was good and completed. Our links are unique to each transaction, only good for the files the buyer purchased, and only for the number of attempts and expiration time period you specify.
Just configure your products with the File Download setting enabled, then upload your file for each product. If you are on a subscription plan of at least $18/mo, you also have the option of hosting your product files remotely, on your own server rather than ours. In either case, our download links would work the same way, with all the same access-control and expiration settings, and our links would conceal the "real" static URL of your files, so those URLs cannot be passed around the Internet nor used for unpaid access indefinitely.
If your product consists of several files, you can combine them into a single archive file using a program like WinZip, and offer that file as your download. If you have several products that you also want to offer together at a special price as a "package deal", you can use our Package Products method:
http://www.e-junkie.com/ej/help.package.php