Feature Request: Dynamic Billing

As a startup our team is very dynamic, we regularly add new teammates and sometimes teammates leave. We hope the billing number of seats could automatically adjust to it too. For example if we purchased 12 seats and one teammate left, the 1 seats should stop charging / accumulate credits.

What I really like is Slack's approach were charging are automatically updated when a new teammate is added and stops charging when the teammate becomes inactive. This make it easy that customer won't have to about seats or billing at all.