When did the USPS start renting out ad space?

I’m referring to the new(ish) “Spirit: Stallion of the Cimarron” postmark that the USPS has been using for the last few months.

AFAIK, the movie wasn’t produced by the USPS, so why is the USPS promoting it?

The obvious answer, I suppose is money. So when did the USPS start renting out this space, and why is Dreamworks and the rather unimpressive Spirit the first to take advantage of it?