Taxes and fees should be mandated for all prices up front. This is how it works in most of the world, there's no good reason it shouldn't work that way here in the US. Someone will come along and point out an edge case, but for the vast majority of retail situations this is doable.
Or just abolish sales tax. If the states want the money, they can just raise state income tax, which they're already charging, by a few percent. As it is now, it just complicates things and disincentivizes consumers from making purchases, which slows down the local economy.
Sales taxes are hard to make progressive (they're usually regressive) and can be harder to enforce (if they're local).
It depends on your goals. It sort of makes sense that we have both, at differing rates.