I also find it luck-of-the-draw. I am charged $1.95 about 80% of the time and $.45 about 20%. I just go with however they categorize it and smile. For the same reason that I don't send back food in a restaurant, I don't anger the person responsible for getting my package from A to B.

Now with that said, I only mail 1-3 items per week. If I was mailing 50 - 100, I might feel a lot differntly.