> One thing that caught my eye was that the other softwares do not have automated tests. How do you know this?

Talking to ex-employees

> Do you consider this to be a USP of your software? I do not think end users will care about whether tests exist or not.

I wouldn't go so far to call it a USP (many payroll softwares will test decently), more a baseline for quality. It's not uncommon to hear of payroll errors when talking to people who run payroll. While end users don't care about tests, they very much care about being paid the correct amount.

These errors seem to fall into two categories: manual entry and software errors. Manual entry errors are by far the most common, and seem to be symptomatic of confusing UI and a lack of validation on input (easy to mistakenly put an extra 0 in a number field, not doing a checksum on an account number, no regex on a national insurance number)

That said, calculation errors do happen. When furlough was brought in, lots of payroll softwares struggled. We think part of that could have been avoided with better testing.