Prince William and Kate Middleton Cordially Invite You . . .

Start watching the postbox! ABCNews has learned that invitations to the royal wedding of Prince William and Kate Middleton went out from the Lord Chamberlain's office on Thursday. Of course, most of the guests already know who they are since many of them received a save-the-date fax from the Queen a few weeks ago. The 1,800 invitees will receive a "traditional, gold-embossed handwritten invitation," according to the Daily Mail.

Several European royals have already RSVPed including one of William's godfathers, King Constantine, the former ruler of Greece. A spokesman for him and his wife Queen Anne-Marie and their son Crown Prince Pavlos confirmed to the Daily Mail that the Greek royal family received a fax and plan to attend.

But the most sought after ticket isn't the one that gets you into Westminster Abbey. Katie Nicholl, author of the book "William and Harry: Behind the Palace Walls" told ABCNews that the "ticket that everyone really wants -- and it is the golden ticket -- is the invitation to the evening reception which is being hosted at Buckingham Palace by the Prince of Wales."

Just 300 people are attending the exclusive soiree -- and Nicholl said "William and Kate have been told they can invite 100 each."