text: if i sent a letter with insufficient postage to a local address, how long will it take for it to be returned? i sent several letters with old 37 cent stamps twelve days ago.  so far, none of them have come back--but they didn't all have return addresses, either.  i'm just wondering how long it would take for them to be returned, and if they are not, can i assume that my local post office is too overwhelmed to check? in australia, if you post a letter with insufficient postage they still deliver it to the addressee. however if your return address is on the back they will send you a bill for the missing postage plus a fee. if there is no return address they still deliver it to the addressee. no one is charged the missing postage in that instance.