All you have to do is turn the situation around on them. Most people are pathetic wannabe tyrants. Any little power goes to their head.

So, just put them in their place. If they tell you to wear a mask, say "I'm unable to wear a mask, thank you." If they press, tell them you have a medical condition that makes it unsafe for you to mask. If they continue to bug you, or ask for details don't engage any further. Just ask for their manager. If they are a manager tell them you'd like to know who they report to. If they ask why, tell them you want to know if its the store's policy to violate the ADA by refusing to provide a reasonable accommodation for your condition or whether that employee is acting in violation of policy by doing so. Tell them you plan to file a complaint with the local state's attorney's office. Then watch them crumble.