Do not overreact at the problem and give ample chance to the outlet that made the mistake to correct it. Generally no one wants to make a mistake on purpose. Let them know about the situation and they should fix it. If not, they will give you a reason. Your next course of action will obviously be based on this reply.
Meet the Manager
If things are not resolved, meet the person in charge. It is most often the manager of the store who has all the experience in the world to deal with such situations and can provide you with a good solution to the problem. This should do it and things should be settled. If this does not change anything, take it to the next level.
Call the Corporate
If you are dealing with a well known or big brand store, call their help line and let them know about the problem that you are facing right now. Explain them the whole issue and what you have done so far to resolve it. They should be able to fix things for you. This will generally take a day or two but the problem should be resolved. If you know that you are right and they refuse to help, it’s time to move to other channels.
Contact a Consumer Agency
These guys are there to protect the rights of the buyers as it is their bread and butter. Contact them with all the details and if your case has a merit, they will certainly help you. The stores will be very happy to fix issues once these guys get involved.
You can also get on to the internet and let the world know about how bad the service has been with you and what problems you have faced. Make sure that you select a forum where retailers also have their people listening to feedback and to fix their reputation, the retailer will most likely work with you to resolve the matter.