FHZM and 小小爱茉莉 are right, that's your mistake.

When you creat shipping from paypal, there is a link to state what the thickness should be -- that is 3/4 inches so that your package can be eligible for the delivery confirmation. When your package is not thick enough, then the package will be automatically upgraded to priority mail letter service, and the postage cost more. Post office doesn't want to waste their labor to return to sender, unless the addressee refuse the package. buyer already paid for the item and the shipping fee you aked for, so she/he knew any addictional charges should be your resposibility, now you refuse to take the responsibility, that's why she/he got mad and left you a negative.
(I experienced such situation and called both my buyer and post officer to clear the WHY, now I won't make the same mistake again)