How these numbers are checked

A wrong calculator is worse than no calculator. This page says exactly where each number comes from and what happens when it changes.

Where the rates come from

Every rate is transcribed by hand from the operator's own published fee page — not from a blog, not from another calculator, and not from a model's memory. Each tool links to its source and shows the date it was last verified against it.

Tool	Source
eBay fee & profit	eBay — Selling fees
PayPal	PayPal — Merchant fees (US)
Stripe	Stripe — Pricing
Walmart	Walmart Marketplace — Referral fees

What happens when a rate changes

A monitor re-fetches each source page on a schedule, extracts the fee section, and compares it with the copy that was in place when the rates were last verified. It raises an alert in three cases, and the third one is the one that matters most:

1. The source changed. The published fees moved, or the page was reworded. The rate tables here are re-verified by hand before anything is republished.
2. The source could not be fetched. Silence is not agreement. A monitor that cannot reach the page is a broken monitor, and it says so rather than reporting "no change".
3. Nothing has changed for an unusually long time. A fee page that has not moved in months is more likely to mean the extraction broke than that the fees are frozen. The monitor alerts on that too, so the check gets checked.

That third rule exists because the failure mode of a rate monitor is not a false alarm — it is years of quiet, confident, wrong numbers.

What the calculators deliberately do not do

• They do not invent rates for anything the operator has not published.
• They do not fill gaps with estimates. Where a fee schedule is out of scope (eBay Store subscriptions, Walmart fulfilment, non-US accounts) the page says so rather than guessing.
• They do not round in the operator's favour or yours. The arithmetic is the arithmetic.

Accuracy

The fee engines are covered by a test suite of hand-computed cases — tier boundaries, threshold cliffs, the sneaker exemption, currency-specific fixed fees, reverse calculations and their round trips. The tests assert values worked out by hand from the published tables, not values produced by the code itself.

Found something wrong? The rate tables are plain JSON and each one names its source and its verification date: eBay, PayPal, Stripe, Walmart.