I've done this myself a few times when selling items... I live in CA, and many of my buyers are in CA. I've discovered that the shipping time for 1st Class and Priority is often the same, while the price for Priority can be double or more. If I see that the buyer is a CA buyer, I will often ship 1st class instead of the stated Priority for this reason. I've never had a buyer complain, but if one did, I suppose I'd refund a buck or two to them.