I don't see a problem with a handling fee to go with the shipping fee... I know when I sell items on ebay i charge both shipping (actuall cost including insurance), plus a few dollars for handling which pays for the box etc. as for gas to go to post office, i swallowed that, usually cause I was going past there anyways. If i had to to go to toronto air port (2.5 hours away) i would charge something for that.
I know when I've bought snakes from breeders that had to ship I paid more then actual shipping charges. usually in the neighbourhood of 20 some odd dollars.
of course it makes a difference of what you're selling... if some one was dropping 20k on an animal i'd probably throw in shipping for free. what's the big deal with $100 on a 20k animal?