Business owners have no legal responsibility to let people in without a mask and it doesn’t matter what letter they have or what medical condition they might have. It’s private property and access to the property is totally on their terms. They can deny someone for any reason. Although they might get sued later if they deny entry to a member of a protected class. Idiots waving “a letter from the Governor” are not a protected class.If you have a medical exception from a doctor on masks you probably have a letter stating it as well. I would recommend taking it with you. BUT...do not be surprised if a business owner denies you entry.