It depends on when the buyer cancelled the transaction, if they notified you within 14 days of receipt, then you have to refund the full amount the buyer paid including any postage charged (but not return postage if it’s a change of mind) this is Consumer law.

Amazon however extend the legal period to 30 days and in some categories offer free returns, so can depend on the category if you also have to refund return postage, however for marketplace sellers, Amazon state sellers do not have to refund outgoing postage if the buyer cancelled between 14 and 30 days.

Refunds

Where you’re cancelling your purchase within the first 14 days from the day you receive, or a third party indicated by you (other than the carrier), receives the last goods ordered by you (if delivered separately), and there’s been no error on the Seller’s part, the Seller will refund the cost of the item and the cost of sending the item to you as follows:

Where you’re cancelling your purchase between 14 and 30 days after delivery and there’s been no error on the Seller’s part, the Seller is only required to refund the cost of the item. Please note that Sellers aren’t required to refund the cost of gift-wrapping, or other services provided to you in connection with your purchase. If you use a personalised pre-paid return label, the Seller will be entitled to deduct the return postage from your overall refund.

Sellers are only required to refund standard postage charges corresponding to the least expensive type of delivery offered by them