Credit: Mario Testino

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Just days after Prince William, 28, and Kate Middleton, 29, selected the wedding party for their April 29 royal nuptials -- Prince Harry will serve as Best Man, while Middleton's sister Pippa will be Maid of Honor -- a St. James' Palace spokesperson confirms to that the invites have been mailed.

"The invitations to the wedding have gone out," the rep tells Us. "By now, Friday afternoon, I can confirm that they will have all gone out by now."

A source adds that the approximately 1,800 invites were sent starting Thursday evening, but because there were so many it took until Friday to get them all out.

As for who should be expecting a thick envelope in their mailbox, an insider told Us in December that David and Victoria Beckham should make the cut along with Sir Paul McCartney, President Barack Obama and First Lady Michelle.