My website traffic was a couple megabytes per month. I paid them for about 3 years for neglegible usage. Should I have zero expectation for paying because its cheap?

You can be metered or fixed price. Mixing the two in the most inuintive way possible is a true innovation. /s

Other cloud based provider have the option to pay for your usage to the cent. I guess that innovation didn't reach the EU. /s

My experience with all EU providers so far is yes, you do have to prepay, I think Hetzner for exemple was 20 euros when I signed up years ago?

Most likely they are unwilling to let customers pay later without any guarantee that they're good faith and solvent. Maybe it is a cultural thing? It's only my personal opinion but it never struck me as unreasonable.

Assuming they've hosted you for 3 full years, I wouldn't mind the 10 euros. Even small sites need all infra to work. The fact that your site is small only means it costs less in transfer.