Overcharged - no customer service

When I got home from shopping yesterday, I noticed that I had been charged twice as much for an item as I should have been. I sent a message to the Checkers customer care centre, received an acknowledgement that they had received it. Now, more than 24 hours later, I still haven't heard from them. Looks like Checkers are in the business of overcharging customers, and don't put any kind of priority on resolving the problems amicably. No wonder their share price keeps falling.