Volotea charged me for fuel

Hello everyone, I’m writing this post just a to ask you whether it’s legal or not what Volotea did to me.

I purchased a flight ticket two weeks ago for a trip from yesterday to this Sunday, and just some days prior to the departure, I received an email from Volotea asking me to pay additional fees to “unlock” the booking. I’ve been charged almost 30€ and since I had to take this flight I payed that, but I’m not sure whether this is legal or not!

What do you think? Is there a way to complain about this?

PS: Volotea really sucks, their app is one of the worst I have ever used.