Given the title of your thread, you should probably read Barry's post about Cites & jurisdiction - since it's not really Cites that you should be concerned about within the USA.
I seemed to recall Barry discussing the issue of postage, but I'm unable to locate the specific thread. What I recall (or think I recall) is that he's discussed the issue with appropriate authorities and the specific wording makes it a gray area. He suggested taking the more conservative approach as earlier respondents in this thread suggest.
Even if you were able to get someone to tell you it's ok to charge postage, there's nothing saying that a year later, someone else has a different interpretation and then uses you as an example.
If you think that the governing authorities are always completely rational & sympathetic, you may not be familiar with the experiences of a forum member a few years ago. Check out the post by Killerplants in this thread (although not completely relevant since it involves importation). I've seen a lot of people post justification saying something like "I did 'X' & nobody said anything so it must be ok." It really doesn't matter how many people got away with something if they decide to use you as an example.