When the prices are listed pre-tax then use the amount of money you have in pre-tax dollars. If you have $40 and the sales tax is 7% then use your phone to divide your $40 by 1.07 and that's how much you have to spend. You don't have to do the multiplication for each item individually and items with different tax rates are typically entirely different classes of product which are sold in different stores.
Traditionally this is called fines rather than taxes. The ostensible purpose of general sales tax is not to bankrupt every grocery.
> this just makes it so you can do simple addition in your head while you walk around the store instead of throwing in some multiplication
Why do you have to do any math in your head at all? The register will do it for you at the end.