Here is a helpful guide on how to make a nuisance of yourself by lying about your medical condition.
Just wear the mask. It’s not that big of an imposition on you, and you spend way more time trying to find a way around it than you would if you just obeyed the order.
Here’s the other thing: There are people who have legitimate medical exemptions to wearing a mask, and when you have a bunch of jerks lying about phantom medical conditions it makes it much more difficult for those people. If enough people do this, stores will just require everyone present a doctor’s note at the door. It is totally ridiculous.