Reply Paid Arbitrage

What charities say on their reply-paid envelopes:

Your stamp on this envelope adds to your gift!

What charities should say on their reply-paid envelopes:

You could put a stamp on this envelope to save us money, but why not donate an extra 45 cents and let us pay the postage, since our charitable institution reply-paid rates are much lower than what you'd pay?

That way our charity gets more money, the post office gets less money, you get a bigger tax deduction, and the government gets less money to start foreign wars and hire meddling bureaucrats to make your life hard!

I guess the people that think hard about  how to arbitrage the government don't tend to end up in the charity fundraising business.

Postal reply arbitrage does have a somewhat checkered past, but I think it's now time to rehabilitate its reputation.