Sorry to hear that, Connemara......of course you would feel down as a result........already suggested....I would go back to the P.O...maybe you just left it on the counter or some good soul handed it back in, if you dropped it there.
Either way it will rest your mind to see if they have it....if not then CCTV would be in there to see if it was stolen, and I would report it to the police too, especially if credit cards were in your purse also, and cancel them straight away, my lovely.
Not ever happened to me, as my wallet is like Fort Knox....I do hope you feel better soon, Connemara and best wishes for Christmas and The New Year
:0) xx