Induce the Usage of Ad Stamps to increase profit for the Post Office.

I have an idea to assist with the Postal Service. Now this idea is to change stamps from normal pictures, like they have now in days into mini ads. Basically, what would happen is a company would place a bid on the rights to produce a set of postal stamps that would be issued for a certain period of time depending on what the details of the bid were. These stamps themselves would have an ad on them as a picture.