She said that it was policy for customers to wear masks
I looked her in the eye and said: I guess I'm not your customer. And walked out.
The look of disappointment on the salesman's face as he saw his commission check being shooed away by the "receptionist" was hilarious! I found a based small business nearby and now I already have the tile picked out.
I have noticed as of late with mask Nazis that it is a receptionist or hostess that gives the moral superiority request AND is exclusively female.
I even had a doctor's office refuse me because I wouldn't wear a mask. Receptionist said doctor wouldn't see me. I replied, "He must have failed basic microbiology and I have no confidence in him now." I left. Keep in mind there is no mask mandate in this area.
As someone here has suggested, try "I'm not allowed to wear a mask as a condition of my parole . . . "