The difference between a number and an answer
Type an amount into a fee calculator and it shows you a number. That number is useless the moment you ask 'how do you know?' A fee depends on the rate, the fixed charge, whether the payment crosses a border, and whether currency conversion is involved. A tool that shows only the result is a black box that happens to be right sometimes.
Surfacing assumptions
Our PayPal Fee Calculator breaks every result into its parts — the percentage, the fixed fee, any international or conversion component — and states the assumptions it made. If it assumed a domestic payment, it says so. If the rate it used came from a published page, it names the page.
'Last verified' as a design pattern
Rates change. A calculator that quietly keeps using a stale number is worse than no calculator, because it looks authoritative. So the tool shows a 'last verified' date and the source it checked. It is a small, unglamorous element, and it is the single most honest thing on the page: it tells the reader exactly how much to trust the figure, right now.
Honesty is a system property
This is not really about fees. It is about a stance: software should show its work. The same idea shapes how our downloader reports progress in real terms instead of a decorative bar, and how the compressor lets you inspect quality loss with your own eyes instead of trusting a percentage. Trust is the accumulation of a hundred small decisions to tell the truth about what the software is doing.