How often is sales tax changing that it becomes infeasible to update the shelf price tags? The grocery stores seem to handle it just fine changing the "buy one get one" type tags every month!
a) Universal sales tax within the state. Allow local governments some other varying taxes (such as income or paycheck tax)
b) Put small bracket under the price “(includes 9.5% sales tax for zip 98210 <a>change location</a>)”
In general though I think physical storefronts would be the shoe in. Virtual might require more thought or even changes to how we do taxes to make as much sense.
Many stores like to pre-tag each product package, either at the store or manufacturer- you couldn't do that if the tax had to be included in the price as the stores have many locations each with different tax rates. (I do think the tax rates should be prominently posted at the door).
Sales tax compliance is already a massive PITA for retail stores. Making them include the sales tax on the tag would just be an even bigger PITA.