Flexport will ask for limited access to your Amazon Seller Central account to move forward with a shipment going to an Amazon FBA warehouse. With limited access to your Seller Central account, Flexport can input necessary information into your Amazon shipment plan (like pallet dimensions), and download FBA and pallet labels as needed. 

See Sharing Seller Central for instructions on how to send the correct permissions to Flexport. 

Note that sharing permissions are only available for professional Amazon selling accounts.