Perché il prezzo visualizzato è diverso da quello nel carrello? (Finnair)

Finnair was known to scam people with prices if they use the English version of the website. If you want the lower price use the Finnish one.

I have no idea if your specific problem is related to that but I think it is worth trying.

In economy class, there are different seat types: “standard”, “preferred”, “comfort”, “front row”. Their costs differ.

Besides the fare class change of 35 eur, there are no more “standard” seats remaining, so now you have to choose at least “preferred”, which costs 12 more euros.