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.
Address labels perform worse than computer printed addresses directly on the envelope.
Make a dummy sample to determine folding of paper, size, and most importantly weight. Postage is very expensive, and if you go over the designated weight set by the post office, you will be paying for it. People like to do things. Checking a box, using stickers and stamps, work to improve response.