Returns Center billing cycle is every 30 days. Your credit card will be charged automatically and invoice will be sent out to your registered Email ID.
You will not be charged, if you are using starter plan and number of returns submitted is up to 3. However, you will be charged for any extra usage.
For example : Plan: Starter Monthly usage: 6 returns.
Free returns - 3
Paid returns - 3*$2 = $6
Total invoice amount = $6