Rolling out two improvements around proration in the API! 🎉

When a customer moves to a higher-priced tier - Proration items will be included on the next charge in their billing cycle. For example, a customer signs up for a $12/mo plan and upgrades to a $24/mo plan a week later. On the next charge, we'll do two things: $9 unused credit from their $12/mo plan, and $18 of usage from their $24/mo plan from the point the plan changed = $33 charge. This is a simple example, we'll calculate down to the second in reality. 

When a customer moves to a lower priced tier -  If the change from one plan to another creates a proration larger than the following charge, or any number after that, a charge will not be created until the credit has been used. This is notable for annual to monthly plan changes, we'll keep track of the balance of the annual plan and resume charges when it has been used.