why are post office rates different?

[Chris142]

the customer sent me a pic of the tracking number on his reciept.it also showed $12.75 shipping as did the sticker on the box plus he sent me$13 to cover the shipping back..i took.it to the post office and told the lady i wanted it shipped back the same way it came @The $12.75 price and she said it would cost $48 to ship it back the same way it came