I have used a wood stove for many years and I have learned that the top of the stack has to be higher than the highest part of the roof. Also some professionals say the taller the stack the better and for every 90 degree bend you lose 3 feet of hight. I've learned that a plain flat cap about 5 inches above the top is sufficient, mine is home made using an 8 inch square peace of sheet metal with 3 peaces of 1 inch metal strips attached together with screws to the top of my stack. It's mostly to keep out the rain. If there is not enough air flow coming into the room where the store is it will definitely smoke back. If there is no restrictions in the stack you can try to open the vent that controls the air flow into the store for a few minutes before you open the door. I'm thinking that something is not right in the store itself. Good luck I hope you can solve the problem soon.
