Requirement:
We want to have a base subscription which is X/month, done via recurring payments, and give clients the ability to choose extra "add-ons" as they please. An "add-on" will be Y/Month and should last the remainder of the base subscription. For example, the base subscription is 10/month and feature X is extra 5/month and feature Y is an extra 7/month. I choose to have the base subscription with feature X and Y so my total monthly payment for a year will be 22/month. Now, if I chose only the base subscription for the first 6 months and want to start using feature X for the final 6 months of my subscription, the final 6 months should be 15/month opposed to the previous 10/month. In further terms, add-on features subscription should last the duration of the base subscription.
Is there a simple way to do this via the PayPal API?