Worse yet, they probably tried to bait-and-switch and sell you brownies instead of brownnies. 
As long as they didn’t give you bronies…
Te first response nailed it (as far as Common Law is concerned):
An Advertisement is NOT an offer to sell; it is a solicitation of an offer to buy.
These is why you can’t rush to a store and get the deal of a lifetime because of a typo in the ad, or note that the ad was not specific to a time and place, and take it from Smallville in 1960 to Megopolis in 2000 and demand they honor the ad.
Yes, you have an ancient ad and you are offering $.15, I decline your offer; Would you like to offer me $10? No? Have a nice day (somewhere else, bozo).
As mentioned, there are local laws about marking up pre-existing inventory (your marked price reflects what you paid for it; you don’t get a bonus for being unable to move a product before the price went up. AIUI, here are places which forbid the marking up - period. You cannot legally put a $30.00 sticker over the old $20.00 sticker.
Now that everthing is scanned and the price set by computer (thereby eliminating 2/3 of a stock clerk’s job), I have no idea if any of those laws still exist or how they are enforced.