You must be kidding me. If you are sick, you are sick. If they tell you to go in and open the store, you tell them "NO." So you are telling me you have to drive a car with a migraine, and open a store with a migraine, and safely check prescriptions and function as a pharmacist with a migraine? That's the dumbest bulls*** I ever heard. When you are sick, you are sick, period. There is no opening a store and waiting for coverage to come in. Once you notify your DM, it's their responsibility to find coverage. If they "can't find coverage", well, they are a pharmacist, right? I'm sure they are capable of opening the store. Please, have a backbone.
I like what the poster above said. If you find yourself going into work even after telling your supervisor you are sick, make sure he or she knows you will only be there so the store can remain open, and that you will not be filling prescriptions. Give them a reason to find you coverage.