A total is not an answer
What building NetPayout, a set of marketplace fee calculators, taught us about showing the working.
NetPayout answers one question for people who sell online: after the platform has taken its fees, what do I keep? It has calculators for 13 marketplaces, and it can put the same sale side by side across platforms.
The arithmetic was the easy part. The lessons were about trust.
Show every line
A calculator that prints only a final number asks to be believed. A seller who sees one figure has no way to tell whether a payment fee, a listing fee or a shipping charge was forgotten.
So every calculator lists each fee on its own line, with the total at the end. The total becomes a conclusion the reader can check, not a claim they must accept.
Use the platform's own pages
Fee rules change, and second-hand summaries go stale quietly. Each calculator is built from the marketplace's own published fee pages, and the data is re-checked against those pages. When a number is wrong, there is one place to look and one source to compare it with.
Compare like with like
A comparison is only fair if every platform is given the same sale: same price, same costs. That sounds obvious, and it shaped the whole interface. One set of inputs feeds every platform, so a difference in the result can only come from the fees.
What we took from it
- People trust a result they can follow more than one they are told.
- Data needs a named source and a habit of re-checking, not a one-time import.
- The most useful feature is often the plainest one: the same input, shown across every option.
These ideas travel well beyond fees. Any tool that turns rules into a number, such as a price, a quote or an estimate, earns more trust when it shows the working.