You're focusing just on consumer/retail/store sort of thinking. The problem is wider than that. Figuring out where someone placed an order is sometimes required, even if it will be delivered or provided somewhere else. Understanding the use case - you may be selling a service, but if the service is performed in different locales, the service may be taxed at different rates. Advertising a service for $399, but performed in different counties, will mean a different tax rate. Now... you can figure it out after the fact, and charge everyone a flat $399, and pay the different taxes yourself (so your revenue might $380, $370, $360, etc), or you add on variable tax based on where that service is done, so the purchaser is paying $399 + ($local tax rate).

Again, that's the business's problem.

Be transparent with the price, tell the consumer precisely what they're going to be paying upfront. If it's hard, too bad, figure it out or don't be in business. At least that's how it should be.