FlyOne Overcharged for Baggage During Check-In Process

I was allowed 5 kg of baggage, so I headed to Chisinau Airport with my backpack. However, they insisted I fit my backpack into a small metal box, even too small for a child's bag, or pay 30 euros. The communication style and customer service were unlike anything I've seen worldwide. Three hours before departure, we joined the check-in line, and when it was our turn, we were told we could only check-in online, or pay 30 euros. However, they didn't provide us with the option to do online check-in. We want to pay, but their website doesn't allow online check-in. They kept directing us back and forth. The payment machine doesn't give change. We barely fit the baggage, yet she still points us out to her colleague, warning that if we add something later, we have to pay her. Authorities need to sanction this company as soon as possible. I've never seen a charge for check-in anywhere in the world.