Maybe just a mistake on the stores part.
Stores no longer put price stickers on items / the price is on the shelf.
In my County, there is a law that says-
if the item rings up higher at checkout, the store has to refund you the error (X10) up to $10.00
In the OP situation, I would have talked to the store manager and taken his word for the pricing error.
If you were polite, you'd most likely get some inexpensive filters.
Depends on the jurisdiction. There are some states or municipalities with laws saying that a shelf price must be honored unless the shelf tag has a clear expiration date. And even then I've gotten a sale price when the shelf tag wasn't removed, but it was at the discretion of the checkout clerk or manager.
As I read my state's law, my interpretation is that if it is sold, it has to be for the lowest price in any ad, shelf tag, or price tag. However, I don't read it as requiring that the price be honored if they decline to sell an item.
12024.2.
(a) It is unlawful for any person, at the time of sale of a commodity, to do any of the following:
(1) Charge an amount greater than the price, or to compute an amount greater than a true extension of a price per unit, that is then advertised, posted, marked, displayed, or quoted for that commodity.
(2) Charge an amount greater than the lowest price posted on the commodity itself or on a shelf tag that corresponds to the commodity, notwithstanding any limitation of the time period for which the posted price is in effect.
I do wonder sometimes about suggested prices by the manufacturer that are on the label. The big one is Arizona beverages, with 99 cents clearly stated on the label. I've paid more, but didn't complain about it.
