Earlier quoted context omitted.
Taxes are set at a multiple levels (federal, state, district, and city) so it's pretty much impossible to show the post tax price on the label. Hidden fees are a completely different issue.
That only makes sense for an online store that's going to ship you something, in which case asking for the destination ZIP code up front solves that problem. For brick-and-mortar stores, the store can quite easily calculate the final price of an item on its own. Sales taxes do change on occasion, but likely less often than the pricing of the items themselves do, so there's no added burden in having to update them due…
Tax boundaries do not follow zip code boundaries.
To actually find the right tax rate you need the full address, including suite or apartment number because there are cases where a tax boundary runs between different suites at the same street address.
Also this is not just for online stores that ship something. It also applies if you are selling a downloadable good like a game or music, or selling memberships to your site.