I'm not sure when they first started doing this, but I do know the USPS has long been in the stationery business, that is to market stamp-themed note cards, stationery, envelopes, etc. Some are grossly overpriced and few consider them a stamp collectible, more appropriately relegated to the "ephemera" category. Some are also sold outside of the USPS at some card stores or Department stores. In fact, after they've been out for awhile, you can occasionally find them discounted in stores specializing in closeout merchandise.
As for the images you scanned, here's a current
ebay listing where the actual set is being offered for sale:
http://www.ebay.com/itm/USPS-Love-S..._1101wt_1011