You're overlooking the context. Someone said it was easy to let young children go shopping in places where taxes are included in the sticker price because you can give the kid $5 and the kid knows they can buy any item with a sticker price that does not exceed that or a combination of cheaper items whose sticker prices do not sum to more than 5$.

The implication was that this is harder for young children in the US, presumably because young children won't be able to do the complicated mathematical finagling that the rest of the world apparently thinks Americans do every time they go to the store.

My point is that in the US you just give the child $5 plus enough to cover tax, and then the child can use exactly the same approach those kids elsewhere use: if sum of sticker prices <= $5 they have enough money.