It doesn't have to be much. It can come with an explanation. If they are learning English, it's useful to know for the anglophone world ghosting (in many situations) is considered extremely rude.

I think Japanese people would be very receptive to this information and the $ incentive to let them know you're serious about it.

Structure as bulk "Buy 10 classes at a discount"

From your username I figure you might know: are any other services sold this way in Japan?

I ask because if not then it's going to be difficult to go first. If yes, any idea how it influences behaviour?

It may overcompensate... Call me cruel but I'd have a very meta chuckle that it was economically worth X to the student to not have to message me!