In a world with tax-included prices, obviously they would advertise the local price in local media. In national media, who cares what they do? They could put "$

In a world with tax-included prices, obviously they would advertise the local price in local media. In national media, who cares what they do? They could put "$0.99 (prices may vary)" or whatever they choose. The headache of an advertiser should be the least of your concerns when dictating consumer law. (And yes, it is law in many countries. Consumer protection laws say that consumer pricing must include VAT.)