Just an FYI, you can get free boxes from the bost office as long as you use them to ship via Priority Mail. In fact, you can order a case of them
from their website and they'll deliver them to your home for free. That should cut the expenses for you a little.
As for charging for your bus trip to the post office, do you make a separate trip for each package you ship or do you ship a bunch at a time. I don't thin you should be charging each buyer the full price of a bus ticket but I don't see a problem if you add a small amount like $.50 per person to cover your expense.