On MUC-HEL, there is a business class cabin where you will be served a cold salad and some alcoholic beverages. You may upgrade with cash, go to Manage Booking on the website and look for a link there. As a rule, upgrades are not worth it.

HEL-KTT (Kittilä is KTT, not KIT) is economy-only.

If your carry-on looks okay sizewise and you do not visit the check-in desk, your risk is minimal. Sometimes, agents at the check-in desk want to weigh a carry-on, but they do not have scales at the gates.