Honey, not vinegar. No flashy pictures - just a thought:
Even though I was first to post about staying safe with the virus, nothing sets me off quicker than seeing a sign saying, "In order to obey Gov Newsom's mandate to wear masks, [blah blah blah]"
How about this: "It's our store policy for all our customers to wear protective masks for the safely of our customers, so we kindly ASK you to voluntarily wear masks to help protect you and your fellow customers.
With the second approach, I happily comply. When it's Newsom demanding it and they go with that - I tell them to go f*ck themselves and I leave never to return.