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Check the store's sales policy, and return policy.
Most stores will permit returns, particularly when you have not taken delivery of the product.
If the confusion was due to the store misrepresenting something, you have a cause of action for "fraud in the inducement" permitting you to "void the sale"
However, if it is due solely to your mistake, this is referred to as "unilateral mistake" and is not a basis to force the store to unwind the transaction.
Most stores will work with you to ensure that you are satisfied with your purchase (if you are still interested in purchasing something from them, just not that item, they may be more willing to come up with a resolution than if you are just canceling the sale - this type of negotiation
assumes that the purchase contract doesn't allow you to simply cancel the sale).