According to the Amazon documentation, you simply need to accept the terms of conditions in your Shipping Services section of Seller Central.

Here is a snippet from Amazon:

Terms and conditions

Before a seller can use any application that uses the Merchant Fulfillment service to fulfill Prime orders, they need to:

  1. Review and accept the terms and conditions of the Buy Shipping Services program.

  2. Review and accept the terms and conditions of any carrier that they want to purchase shipping from.

Sellers can review and accept terms and conditions on the Manage Shipping Services page of Seller Central. Calling the Merchant Fulfillment service on behalf of a seller who has not accepted the terms and conditions of Amazon and the carrier returns the TermsAndConditionsNotAccepted error. 

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