I've never worn a mask. You don't have too, either. There are three files below in Microsoft Word, Apple Pages, and PDF formats. Use the PDF format if you're in Alberta, Canada. Use the Word ( .docx) and Apple Pages ( .pages) files to format to your location.
When asked why I'm not wearing a mask, I nicely say, "I'm legally/medically exempt from wearing one."
Then I hand the security guard or manager the 3 pages, "If you have any questions, please let me know." Then walk away and do my thing.
Sometimes they say, "It's store/business policy that our customers wear masks."