Boy that manager must of never heard of customer service.
Don't let it go because if she treats you that way she will treat everyone that way so you will be helping other if you let them know. Also it would be better to give you the discount and let it go than to lose you as a customer forever. You know sometimes you have to bend a little to keep your customers happy and 20$ would not put Hancock's out of business I am sure. PLEASE contact a District Manger and let them know about this incident.