If you're set up to use PayPal, FetchApp will provide you with a unique link for every product. You can then place that link on any website, blog, social network, email, or virtually anywhere else. When somebody click any of these links they'll immediately be taken to the appropriate processor to collect payment. Once payment has been made FetchApp will automatically deliver a secure link where your customers can download the files.
If you're using Goodsie, Shopify, BigCommerce, or any other system via the FetchApp API, buyers can place orders from your shopping cart. After an order is placed FetchApp is immediately sent the order information from your cart, and then the goods are automatically delivered to your customer via email.