I want to offer my customers the ability to have a Monthly Subscription, or a Yearly, then possibly a 2 year, 5 year and then a Full (Lifetime) options that when the customer checks out a script is send me an email notification and the automatically assigns the customer a serial number from my database while also saving the corresponding expire date on my database for what the user purchased.
With all first time customers. My website would have a "Select Now" button that the customer would add to cart and check out using PayPal. When they checkout with PayPal, I automatically receive an email that I have a valid sale and get the customers info etc. When the customer completes the sale that there is a way to customize a script that would automatically send the customer an email with the direct download link from a secure page on my site then they can download their program, and in that same email, it would pull their Serial number from my Database. In the database, it would be nice if it could associate their customer name with the serial number for my reference.
With all Renewing customers, this may require a similar interface (which I will design) as the first time customers would encounter. However it would need to automatically renew their existing serial number. Then when they renew, the existing customer would use the same serial number, and it would renew the appropriate date.