I work for the Post Office..and sorry to say you are SOL! The carrier should have picked it up, you can call the PO, but they will not be able to post mark it for an earlier date. All you can really do is bitch at the superviser. He will then go to the carrier, and laugh and joke around about the carrier being a dumb ass for missing it. As bad as is is to say that, the Union has us covered for ANYTHING other than stealing mail (and has us covered for that too sometimes).
More than likely it was an accident...we get rushed around while we are on the street...we have a VERY tight schedule to keep and do sometimes forget basic things. - Not that any of that helps the situation....good luck!