William George Auctions
The note written on 21 June, 1989, has now gone up for auction in the UK and includes Princess Diana's signature.
Adding to its sentimental value, both Princes William and Harry - then aged seven and four - also added their monikers to the bottom of the message.
A second letter written by Princess Diana's lady-in-waiting, Ann Beckwith-Smith, is also included in the auction bundle.
The note reads: "The Princess of Wales was enormously grateful to Sergeant Plumb and his team for ensuring that Prince William's seventh birthday was such a very special occasion and the display was enjoyed by grown-ups and children alike."
"Her Royal Highness was particularly grateful to the 'team' for being able to fit this is on a day already full of official engagements, which was not helped by the rail, underground and bus strike," she continued.
"It was greatly appreciated and I would be grateful if special thanks could be passed to all concerned, but especially Sergeant Plumb."
The highest bid currently stands at £7,500 (approximately $14,695).
