The fact that they are considering taxes may also indicate that they do not know/trust you. And it's normal. As an entrepreneur, you never know when the government will send its dogs to tear you apart.
In a situation like the one you described, the easiest option is to explain to the merchant that if they want to pay tax so badly, they should take _n_ fiat out of their wallet and put it in the cash register.
That is, hypothetically, you bought a haircut service for $50, sent $50 to any cryptocurrency that the merchant likes, and the merchant takes $50 out of his wallet and puts it in the cash register. This way, everyone will be happy:
* you (the customer): pay for goods/services with crypto
* the merchant: sells you a service for crypto, thereby opening the door to other people who want to pay with crypto; and the fact that they put their $50 into the cash register = nothing changes for them in terms of accounting/taxation
* the state: robs the merchant of taxes