This is quite tricky if you live somewhere that charges sales tax.
(VAT is a bit different from the way the US-style sales tax for other reasons: https://en.m.wikipedia.org/wiki/Value-added_tax )
It's difficult, but everyone in a local area knows already.
Many (all?) European countries include VAT in the price, but prices of 9.99, 0.99 etc. are all too common.