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.

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.