Folks who do direct mail fundraising for nonprofit organizations can -- and should -- take a lesson or two from commercial copywriters. Among them is the use of the guarantee. Practically every piece of commercial direct mail carries some kind of guarantee, yet what can a nonprofit letter offer? If the donor doesn’t like what we do with his money, we’ll refund every penny? Hardly. But you can adapt the guarantee to offer another kind of protection that reassures the donor his money is put to good use. One way is to tell how each dollar will be spent -- what