~~~ affixed postage since they accepted the piece and agreed to attempt to deliver it at a fee that they charged the sender and has been paid for by the sender.

Though I have not read their regs, it seems as though the USPS meter strip is a legal document since they're requesting the sender to give them the funds they're asking for in order to begin the process of delivery (unless there is something in writ stating if they are in error by not asking for the correct postage fee then additional postage must be added on to the original postage at destination) and in turn the sender receives a receipt of payment in full.

Of course I do not expect the USPS to deliver any piece of mail if the sender did not affix the proper postage.