Everybody IN the store: draw and demand he drop it or get out, then if he leaves, lock the door behind him and call 911. If he declines to drop it or leave, and advances toward you, fire at will (while ensuring a safe backdrop).
He's outside the store, I'm inside: Lock him out and call 911. If he breaks down the door, draw & fire at will.
ETA: I should clarify that in the first instance, a very clear plan for trying to subtly position one's self in a manner where the suspect would be alone in the line of fire, and not have an easy opportunity to take someone in a physical "hostage" position would be imperative.