Actually, sales tax can differ substantially from one locale to another. Different towns in the same county can have their own additions to the basic tax rates, depending on the different levies in place. I don’t do IT in retail stores, but it may be more of a chore to automatically add these amounts than it’s worth when the system can simply do a calculation to add the proper amount to a total.

In my area, a good rule of thumb is to add 10% to any taxable (non-food) item. The actual rate may vary a little up or down from there, but it gets you to within the ballpark (another Americanism).