I will admit that this is a paradigmatic ethically ambiguous situation involving the free market, huge corporations, etc.
if you want to end the conversation, that's fine, but don't cop out like that. you know it's a simple matter.
Bravo. There is really no complexity at issue. Unless the establishment clearly had a policy of providng a "bonus" to induce your very presence in the store, the discount was intend as a reward for purchasing those three items. At the end of the process, you only bought one item and was, therefoe, entitled to only 10% of the purchase price of the one item. Accordingly, I agree with the earlier poster that the correct course of action would have been to bring to the attention of the retailer the error and give the store the option of correcting or waiving the correction. A far more honorable choice than relying on the error of a clerk to reap a financial reward to which you were not entitled. The ebay misspelling example cited is inapposite. The seller has the obligaton of correctly listing describing, and pricing the merchandise. You have no obligation to either correct the errors in listing, description, or price. Similarly, if you choose to overbid or overpay for an item (see, e.g, purchasing automobiles and stereo equipment), the seller has no obligation to correct your error. That being said, irrespective of the refund question, it appears you scored a nice bargain.