I would fully accept a law that allowed companies to advertise nationally the pre-tax price with a small disclaimer saying applicable taxes vary by location, and then require the in store price tags to reflect all taxes.

There’s no excuse not to anymore. Yes it does vary, and a computer can easily adjust the price.

Exactly, I’d argue kids are the most likely to go to lengths to circumvent the rules

Forget tax havens, eventually some countries will probably become content havens and sell server space hosted there. Probably some carribean island