Amazon's referral fee is a percentage of the total price the buyer pays, including any shipping and gift wrap you charge, with a $0.30 minimum in most categories. The default is 15%. Some categories switch rate by price: clothing is 5% up to $15, 10% up to $20, and 17% above; baby and beauty products are 8% at $10 or less and 15% above. Others, like jewelry and watches, charge a lower rate only on the portion above a threshold.
Your selling plan adds a second cost. The Individual plan charges $0.99 for every item sold. The Professional plan is $39.99 a month with no per-item charge, so the calculator divides it by the units you expect to sell. From 41 units a month, Professional is the cheaper plan.
For FBA, Amazon also charges a fulfillment fee per unit based on size tier, weight, and price. Amazon publishes the US fee table only inside Seller Central, so we have not stored numbers we could not verify. Look up your product in Amazon's public Revenue Calculator and type its fee into the FBA field; the result stays right when Amazon changes rates.
Books, music, DVDs, software, and video also pay a $1.80 closing fee per item. Profit subtracts your product cost, the shipping you pay (postage for FBM, inbound freight per unit for FBA), and prep or packaging.
Fees = the greater of referral % x (price + shipping) or $0.30 + plan cost per unit [+ $1.80 media closing fee] [+ FBA fulfillment fee you enter]