About Net Margin Lab

Net Margin Lab builds fee and profit calculators for people who sell on marketplaces. The premise is simple: the sale price is the number everyone knows, and the margin is the number almost nobody can quote accurately. That gap costs sellers real money, and it does not need to exist.

Why this exists

Marketplace fees are unusually hard to reason about. They are charged on different bases — a percentage of the sale price, a flat fee by size and weight, a charge per cubic foot per month, a share of ad spend — and several of them change with the calendar or with the classification someone picked when they created a listing. Holding all of that in your head while deciding whether to buy inventory is not realistic.

The consequence is visible in the market research. Repeated surveys of Amazon sellers find that a large share cannot estimate their own profit margin to within five percentage points. When you cannot state your margin, you cannot price, you cannot negotiate with a supplier, and you cannot tell a good product from a bad one until the bank account says so.

How the fee figures are sourced

• Default rates come from the published fee schedules of each marketplace for the US market, plus published customs guidance for import calculations.
• Every rate is editable, because fee schedules are tiered by category, seller status and marketplace, and no single default will be right for everybody.
• Each calculator page states the schedule it reflects and the date it was last reviewed.
• Pages are re-checked when a marketplace announces a change — for example the 3.5% FBA fuel and logistics surcharge introduced in April 2026, and the suspension of the US $800 de minimis exemption.

What these tools are not

They are planning tools. They are not accounting software, not tax advice, not customs advice, and not a substitute for the fee preview report in your own seller dashboard. Fee schedules are applied per marketplace and per account, and customs treatment depends on the classification and origin of the specific goods moving. Treat the output as a well-founded estimate, confirm the exact figure at the source, and never make a purchasing decision from a single number on a web page.

Nothing you enter into a calculator is transmitted anywhere. Every calculation runs in your own browser.

Independence

Net Margin Lab is independent and is not affiliated with, endorsed by or sponsored by Amazon, Etsy, eBay, Walmart, Google or any customs authority. Marketplace and company names are used only to describe what is being calculated. The site is supported by advertising, which is clearly separated from editorial content and from the calculators themselves.

Corrections

Fee schedules move faster than any site can. If you find a rate that no longer matches your dashboard, that is genuinely useful — please send it through the contact page and it will be checked and corrected.