A referral fee is the commission Amazon takes on every item you sell on its marketplace. It is charged whether you ship the order yourself or Amazon ships it for you, and it is the one fee that grows in step with your price.
Each category has its own rate, and you pay that percentage of the total sale price or a fixed minimum per item, whichever is greater. On amazon.com most categories charge 8% to 15%, a few charge more, and most have a $0.30 minimum. The total price includes what the buyer pays for shipping and gift wrapping as well as the item, according to Amazon UK, where the usual minimum is £0.25. Some categories are tiered, so one rate applies up to a price point and another above it.
A $30 item in a 15% category costs $4.50 in referral fee, well above the minimum. On a $1.50 item the same rate gives $0.23, so the $0.30 minimum applies instead, which is why cheap items carry a much higher fee as a share of the price.
Tax changes the base outside America. Prices on most other Amazon stores include VAT or GST, and Amazon UK says the fee is a percentage of the price excluding taxes, so a registered seller pays the referral fee on the tax-exclusive amount.
Do not mix it up with the selling plan, which is a flat $39.99 a month on the Professional plan or $0.99 per item sold on the Individual plan in the US, or with the FBA fulfilment fee, which pays for picking, packing and shipping and depends on size and weight, not price.
The Amazon FBA calculator applies each store's category rates, tiers and minimums, strips tax first where the store's rules call for it, and shows the referral fee as its own line.