Hey LD.
If they sent you a check, tell them you cant find it and to check their checbook to see if someone cashed it.
They should probably offer to cancel that check and send you a new one, or apologize they may have forgotten to include it.
I have sent large amounts of mail out and then wondered if I put all the necessary things in the envelopes.
I would not count on the post office locating it.
Mazel Tov and Hatzlacha on your marriage!! All the best!!