If you are paying return shipping, a Prepaid Shipping label would be your choice. If the buyer is paying return shipping, Amazon will provide them with an unpaid label which they can use to send the item back.
If you choose a Prepaid Shipping label, you can purchase one through USPS Click & Ship for Business or PayPal. You can then message it as an attachment to the buyer. If you want to pay return shipping, but prefer an easier way, you can choose unpaid label, and refund the buyer their shipping, through your seller account.
Vicky