Being originally from England, I'm used to weighing all my ingredients when baking or cooking rather than the "cup" method.
That being the case, I have a good kitchen scale which is extremely accurate (it's electronic). I pack and weigh at home, print out a shipping label and put it out for the mailman. If it won't fit in the box I leave it double-wrapped in plastic shopping bags and hang it from the "flag" and put a note in the box asking him/her to take it.
I've never had a problem with the weight/postage being incorrect. And, as another poster said, you get tracking online all the way to its destination.
And if you buy a kitchen scale, who knows you may find you like it for actual kitchen use!!