Can USPS refund me my money?

Okay I make a sell jewelry (the things are make are small enough to be put in a big envelope) and have went to the post office numerous times and have been told it can't go in a mailbox that has the small slit but to put it in the big blue USPS box so that's what I've been doing and i've been having customers emailing me that they didn't get their items or they have been damaged is this USPS fault or mine? They told me to ship it this way so shouldn't they re inverse me for foul advice...