I too have had the same experience. Think about upgrading services (and appropriately provisioning services while billing correctly and adjusting the recurring

I too have had the same experience. Think about upgrading services (and appropriately provisioning services while billing correctly and adjusting the recurring billing), downgrading services, recurring billing for monthly payments, auditing all interactions with whatever payment API you're using, then retrying API calls when appropriate. It's getting to the point where most of the man hours in my project have been on the billing system.