I wonder if they would prorate? I ended up choosing proton as a better fit for my needs (since I needed VPN anyway, and I am a total beginner). They are also running a deal and I ended up upgrading and getting a prorated discount and a credit on the account. The folks at tuta seem pretty down to earth and interested in building a good name.

If they do end up giving you a refund/credit/upgrade, let us know!