I was under the impression that if they accepted the package - there was no other charge.
I print out all my labels thru Paypal, even if someone sends me a check for an item. I put the red flag up & leave a note in my mailbox asking the mailman to come to the door to get my packages. He gets them & scans the bar code for each package.
I've never had a problem with the depth of my Flat Rate Packages (knock on wood)- I know they have cracked down on the thickness of the envelope - but I was told, if you can close the envelope - using their sticky strip, it can go. But I do tape around all the edges & around the length and width of the envelope
Guess I have been lucky