I'm Lucy, and I'd be happy to answer your questions today.
There is no specific criminal law that applies to this situation. However, a contract requires a meeting of the minds, and an agreement to purchase something is a contract. If you had no idea how much an item would cost before agreeing to the purchase, and they did not tell you, then you could argue that there was no agreement. In that scenario, they would have to accept the medication back and give you a refund. That's something you could have insisted on at the time this happened, before signing the receipt and before leaving the premises. If you took the medication and left, and have started to take the drugs as prescribed, it would be much more difficult to argue that you did not agree. Signing the receipt is an agreement to pay, and the receipt has the amount on it. The agreement is also implied in your act in taking the drugs and leaving the facility. So that makes it much more difficult to argue that they need to give you a refund.
The best way to avoid this situation in the future is to refuse to hand over your credit card until the merchant has stated the exact amount that will be charged. But I'm sorry to say, it could be difficult to get your money back in the scenario you've described.
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