Selling digital goods is trickier than subscription sales. For one, the user normally downloads a digital asset after making their purchase. This may be a music or video file, but it could also be a software download, a serial number and/or a registration key.

Providing downloads complicates the matter in part because of potential for delays in payment authorization. When selling a physical product, there is usually time between the purchase request and packaging and shipping. This interval allows the payment processor to authorize and charge payment.

Most users expect a digital download almost immediately, leaving us little time to verify whether the user's payment is legitimate. This opens the possibility for fraud or other failure states. Fortunately, the big payment processing services have taken many steps to solve these problems with little inconvenience to the customer or our application's usability.

After purchase, customers are typically given a special URL download link, either in a response web page or through an e-mail. They can then download their purchase from this link. This necessitates a storage scheme where only authorized, paying customers are able to download content.

It's also helpful to provide customers with the fastest possible download, not just for their immediate convenience, but also to provide confidence in the service, encourage repeat sales, and to cut down on support costs due to failed downloads. A Content Delivery Network (CDN) is one possible solution to this problem, which we will discuss later in this chapter.

Finally, there is the issue of securing the download. Often this involves providing a special, one-time download link to the customer. This link ensures that our digital good is only available to paying users. We will explore one method of doing this using Amazon's S3 file storage service and their CloudFront content delivery service.