Tax Avoidance/Evasion

As alluded to in the article Income and Expenses of landed nobles, taxes in townships and cities is complicated and sometimes arbitrarily.
Without getting too deep in the weeds, a rough guideline for urban taxes is below.
Tax System is
Generous <2%
Light 3-7%
Normal 8-12%
High 13-20%
Oppressive 21%+
You could slap a surcharge on whatever the PCs spent while in town, but most Game Masters and players don't want to get that deep into the weeds, but for simplicity you can assume that any tax of 10% or less is already factored into the cost of the goods or services provided and not worry about it. If you already rolled for haggling, you can assume negotiating local fees is part of the process.
If there are bridges or gate tolls, it's probably a couple copper for the entire party at worst, which most player characters don't even blink at even if it is a big deal for local peasants.
Then as a Game Master, you only need to bring up local taxes if you want to showcase how a location's taxes are especially high.
Course if you want to avoid or evade taxes, have whichever character in the party is acting as the frontman for the party's purchases roll Wits + Politics or Wits + Commerce to avoid paying fees.
Difficulty 5 is you are quietly passing through and just buying basic food and lodging. Difficulty 8 if you bought a magic item or otherwise made your party look loaded.
Botch: You angered the tax man and you had to bribe your way out for 30% more or you got arrested and then have to bribe your way out or flee.
Failure: You mildly annoyed the tax man and probably have to pay 10% more.
One success: Avoid a quarter of taxes and fees
Two successes: Avoid half of taxes and fees.
Three: Avoid ninety percent of the taxes and fees (the 10% you paid was probably a bribe).
Four+: You avoided the local taxes and fees altogether.