AWS Billing and Cost Management
User Guide (Version 2.0)

Effective Billing Date

When the member account owner accepts your request to join the organization, you immediately become responsible for the member account's charges. If the member account joins in the middle of the month, the master account is billed only for the latter part of the month. The member account's original owner is still billed for the first part of the month, as shown in the following diagram.

                Who gets billed based on effective date