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.
UPS and USPS 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:
UPS Terms and Conditions
USPS Terms and Conditions
USPS First Class mail will ship most items under one lb. UPS Ground will ship items between 1-150 lbs.
Packages with an actual weight of more than 150 lbs, or that exceeding 108 inches in length, or exceeding a total of 165 inches in length and girth [(2 x width) + (2 x height)] combined, as measured to determine their billable weight, are not 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 in each weight band.
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