Goods distibuted in samw geographical area generally have similar prices even after transportation, so shop in county A that has lower tax, will be cheaper than

Goods distibuted in samw geographical area generally have similar prices even after transportation, so shop in county A that has lower tax, will be cheaper than same shop 1 kilometer (1 mile) in county B, that has higher tax. Why would the shop want to hide the tax? In EU shops are made to show price per kilogram / liter too.