As a seller in the Amazon Prepaid Return Label Program, in addition to Amazon's terms and conditions, you are required to agree to the carriers’ conditions.
Royal Mail and Hermes provide all return shipping for Seller Fulfilled Orders through Buy Shipping services. By using Buy Shipping services, you acknowledge and agree that your purchase of return shipping is governed by and subject to the terms and conditions of the applicable carrier or service provider:
Royal Mail Terms and Conditions
Hermes Terms and Conditions
Packages exceeding carrier permissible weight, dimensions and volume used to determine their billable weight, will not be accepted for transportation. They should not receive an Amazon-generated prepaid label. See Shipping costs for seller fulfilled returns
for the cost of return shipments.
Amazon will charge your seller account for the cost of the return shipping once the customer ships the product. For details on how to handle return shipping cost when issuing refund, visit Refunding shipping cost