It’s more that there was a standard unit of accounting (livres, sous, and deniers) and everyone could convert from one currency to that standard and back to another currency. It moved a lot slower than modern foreign exchange so except for local fluctuations, it was rather predictable.

> were literally worth their weight in gold (or silver, copper, bronze, whatever), so it was probably easier to pay with foreign currency than we might assume

Are you sure you know what the coin paid you is made of? A merchant of the time wasn’t. Those who care not to be scammed have never found it simple.

https://en.wikipedia.org/wiki/Debasement