Want to stop the demands you wear a mask?? Do this. Buy a pulse oximeter from Amazon, Ebay, where ever.  They cost around $25.  Get it and note your normal oxygen level without wearing a mask.   Then carry the little device with you when shopping.  Take readings while shopping.  As your level drops, especially if it goes below 95%, ask to see the manager or pharmacist.  Show them the level and ask them what to do.  Some of these devices come with a chart that shows normal to dangerous readings.  Let them see the device while it's on your finger.  If your levels are in the problem range...below normal or worse, in the dangerous range, ask them what you should do?  Most likely they'll want you to leave.  You can ask if they provide oxygen for their customers?  Obviously, that will surprise and scare them.  None of them thought this through.  

If they tell you to leave, ask if they have any policies regarding people who can't wear masks because of drop of oxygen levels?  They'll probably say no but it would be best if you left.    Get the name of the person you spoke with and then contact the county health department to see if they can help.  They won't.  So, then call a personal injury lawyer.   If enough people do this they will see the light.  They are creating a very unhealthy situation that puts people at risk of hypoxia.  Those with heart or lung conditions could die.  Kids could be impacted with a developmental disorder due to lack of oxygen to the brain.  I'm not joking.  They are playing with fire.

There are enough hungry attorneys out there that would love to file a suit against a big box store on contingency.  The claim would probably get settled out of court and you both will walk with cash.  Or, it could be part of a much larger class action suit where these stores are guilty of providing an unsafe, unheathy environment for their customers.  A big OSHA violation.   Want to give WalMart or Target a big headache?  Watch them when served with a big class action suit.  Fireworks.    Just let us know before you do so the rest of us can short their stock.