Dear Eager. After 1 week of searching and testing, I found that Prestashop version 1.5.4.1 is the best to sell digital goods. To make it more simple, use Express Checkout by PayPal and not to forget to use 1 page checkout on checkout module located at Order Preference. I try using Opencart but need to adjust max files limit which I failed to edit. Not to forget AbenteCart and CubeCart which AbenteCart similar to Opencart and CubeCart you need first to upload and then to locate file location which I feel uncomfortable. Other than open source, you should try XOO and Digital Download Pro that you can purchase them at CodeCanyon. I hope this might help. Thanks.