your store, you have to make it worthwhile for them to offer your goods to their customers. 20% off allows contractors the ability to charge their customers full price for an item and still make a little profit.
Are you sure the contractor isn't going to charge you $2,250?
He may make it up elsewhere (i.e. the cost of fabrication), but we are taking the $1,800 price out of the allowance in his bid for countertops - not the $2,250 price.