It takes ~2 weeks to resolve and will cost you tens of Euros. The biggest threat is that they won't want to deal with you anymore, at least for a while. Happened to a flatmate of mine 15ish years ago.

I'm certain that it also involves receiving and sending a letter.

Letters are why it takes so long, yes

Pray its not a fax.

I wouldn’t be afraid, but I also wouldn’t reverse a legit one. When you do that, you get sent to collections, which makes sense.