Maybe the solution is a truth in advertising law: the headline price always has to be the complete total, unless you intentionally add unusual extras.

Restaurants are in one physical place, taxes aren't a mystery, there's no real excuse for the price printed on the menu not to be the actual price. Encouraging your employees to make side-deals with customers for cash... maybe we should start treating that as tax-evasion on the part of the restaurant. Putting a guess down in the books isn't OK for any other transaction.