As the article suggests, this has been talked about now and again for several years (decades) that I can recall. There were a number of ideas floating around, as I don't think the intent was to offer an advertising "stamp" but rather advertise on the selvage or the reverse backing paper or the booklet area of a given stamp.
There were also other ideas floating around about "billboard" type advertising on mail trucks and/or mailboxes, and even the logo of an advertising partner on postmarks, too, as shown in this proposal that went nowhere:
http://blog.uspsoig.gov/index.php/2...ise-for-you/Although the entire concept may generate revenue, one must remember that because of the USPS being under the control of the US Government, there are political ramifications involved that would not make this an easy move.
Besides, would you really want to see this sort of a souvenir sheet in the US? The two stamps on top are for legitimate use; the Coca Cola advertising below is not a stamp at all, just a "label". Such confusion would be bound to cause problems with the general public:
