What happens if you click on "More..." and "My wallet..." and "buy credits?"
On Italki you don't pay teachers directly. Italki manages the payment process. You pay Italki. Italki says "You have 25 credits in your wallet." You exchange your money for Italki credits. When the lesson is complete, the teacher gets Italki credits. They can exchange them for their own country's money.
This has some good features. The teacher is protected because the money for the lesson will be paid. Italki has collected it in advance. You are protected because the teacher does not get the money until you tell Italki that the lesson took place.
It also has bad features because you need to trust Italki, and because Italki takes (raspar?) some of the money for themself.