this is amazon help page informaiton below on refunds

BMVD is used as an abbreviation for "Books, Music, Video and DVD" products.

Refund non-media orders

When you create a full refund for a non-BMVD product, Amazon retains 20% of the original order-related fees, up to a maximum of £5, for each line item in the refund. This amount is held back as a refund administrative fee. We will credit you with the amount of the order-related fees paid by you to us that is attributable to the amount of the customer refund, less the refund administration fee.

When you create a partial refund for a non-BMVD product, Amazon retains 20% of the original order-related fees, up to a maximum of £5, for each line item in the refund and credits you with a proportional amount of the Referral Fee originally charged for the order based on the percentage of the item price refunded to the buyer.

If you refund a single item in an order, Amazon retains no more than £5 as a refund administration fee. If you refund two different items in an order, Amazon retains up to £5 for each item, for a maximum of £10 as a refund administration fee. Because the formula is applied to each individual line item in a refund, when you refund multiple quantities of a single item, Amazon retains no more than $5 for that line item as a refund administration fee.