A store can't raise the price of an item once you have paid for it. If they accept your check, cash, or put through a credit card charge, you have a completed written contract, regardless of whether the item has been delivered or picked up and whether or not it is actually in your possession.
If they continue to pursue the issue, send a certified letter, explaining that you will sue them in small claims court. If they return your money, as in, fail to deliver the item and instead send you a check for the amount that paid, you can sue them in small claims court for the difference between the amount of money that they requested from you for the item and the amount you actually paid for the item.
You are entitled to what is called the benefit of the bargain. This means that if the store breaches the contract by refusing to deliver on their part of the bargain by giving you the promised item at the contract price, the court will put you in the position you would have been in had the store honored their promise and delivered the item. Had the store honored the cotnract you would have been in possession of an item that, although you paid a smaller price, was actually worth more. Therefore, the court will award you the profit you would have made at the time of the exchange, the money ahead you would have been after the exchange.