2) Do I have to pay to check in online?

No - TUI's online check-in is completely free to use and can be accessed via our Flight Extras website. It also includes the following airlines: Air Europa, Albastar, ASL Airlines France, ASL Airlines Ireland, Evelop Airlines, Germania, Norwegian, Travel Service, Volotea

Flights with other airlines
Flybe, easyJet, Jet2 and Aer Lingus offer online check in free of charge. That said, you'll need to pay for online check-in with most other airlines.