There is no limit to the things you can test via direct mail. If you are testing price, make sure that everything else is constant. Use the same list and the same direct mail piece, with just the price changed. That Work Remember that direct mail is a personal medium. The more personal you can make your mail look, the better response you will get.
Stamps work better than metered mail.
Stamps look more like a personal letter. Flamboyant color just for the sake of color does not pay off. If you decide to use some color for conservative enhancement, browns and greens do not work as well as aquamarine blues, cold and warm grays, warm reds. Soft white book and antique-finish papers work better than slick superwhite paper. Cheap thin paper makes the product look cheap.