There was a pictorial postmark available for one of the books being released a couple years ago. I believe is was titled "owl post station" I do not think it specifically mentions Harry Potter or any other trademarked characters. I remember coming across it one time in the USPS postal bulletin's philately section. I am a big fan of the Harry Potter books, I however did not request that postmark. Try doing a google search or an
ebay search for a cover. I recall seeing some in auctions one time.