A fulfilment fee is what Amazon charges, per unit, to handle an order for you under Fulfilment by Amazon (FBA). You send stock to Amazon's warehouses; when an item sells, Amazon picks it, packs it and ships it, and the fee pays for that work. Americans spell it fulfillment, and sellers often just say the FBA fee.
Unlike a commission, it does not depend on the price. Amazon UK describes it as a flat fee per unit based on the product type, its dimensions and its weight, and the US pricing page says the same: the fee varies by size and weight. In practice each store sorts products into size tiers, and each tier into weight bands, so a bulky item can cost several times as much to fulfil as a small one, even if it sells for less.
Because the fee is fixed while the price is not, it weighs hardest on cheap goods. A £3 fee would be a tenth of the price of a £30 item but a third of a £9 one.
Several costs sit beside it and are not part of it. Storage is billed separately each month, by volume, and costs more from October to December. Since 17 April 2026, Amazon UK adds a 1.5% fuel and logistics surcharge on top of fulfilment fees in Britain and several other European stores. The referral fee, Amazon's commission on the sale, is charged as well, whoever ships the order.
The Amazon FBA calculator finds the size tier and weight band for the product you describe, names the band and those either side, and shows the fulfilment fee next to the referral fee so you can see which one is eating your margin.