Confessions of a Brand Evangelist has a pretty good idea:
Let me suggest a new paradigm. Charge me for postage. Build it into your business model. Sneek a few extra pennies into that cover price. Squeeze a buck out of me with a higher interest fee. I won’t notice. I promise. But here’s what I will notice: when I turn your envelope over and it says “No postage necessary in the United States.” Oh that will be sweet. Not because I didn’t have to pay the 37 cents. But because I didn’t have to pay with my time and attention.
This would have to be balanced by the fact that if I pay my bill online I shouldn’t be charged for postage.