Because the drinks go to the wholesale market to bring much cheaper. Even if a group of friends to basically not bring their own. Or go downstairs to the kiosk to get 2 boxes,

I think it is better to adjust the cost of drinks. I personally feel more than outside 2 to 3 dollars is okay. Some are double that. So it's hard to get people to accept it.

Or you charge a service charge, it is reasonable to charge a small service charge for bringing your own drinks, depending on the consumption concepts of the people in your area.

The last thing you can do is to put a sign on the door that says "no BYOB"