What it teaches
Subtraction inside a context, which is where it becomes useful rather than a rule. Change from ten is the sum adults do most often, and children meet it long before they are asked to do it on paper.
It also rehearses counting on โ working up from the price to the amount paid โ which is faster than subtracting and is what most people actually do.
What to look for
This one is worth doing away from the screen too. A real shop, a real note, and the child working out what should come back is the same exercise with much more attention behind it.
If the subtraction is the obstacle rather than the idea, go back to Adding and taking away to 100 for a while.