We are currently aware of an issue for some orders where errors are returned or carriers aren't available for Amazon Buy Shipping. Amazon has updated their status page to reflect minimal errors for merchant fulfillment and can be followed here:
http://status.mws.amazon.com/We will provide updates as they are available and apologize for any inconvenience. Until this issue is resolved on Amazon's side, we recommend shipping using a different carrier or shipping within Seller Central.