USPS rules require bi-fold and tri-fold self mailers to be tabbed to qualify for the lowest postage rates. 1 tab is allowed if the weight of the paper is 28/70 (28# bond/ledger = 70# offset) or more and the fold is below the address (open edge above the address). A lower weight or a fold above the address (open edge below the address) requires 2 tabs.
Mistake 1 is to save a little money on paper (20/50 copier paper or 24/60) that results in additional tabbing cost or higher postage.
Mistake 2 is to design a piece with the address in the wrong place. Folks often put the address on the bottom panel of a self-mailer or newsletter. The folded piece then has the fold above the address and the open edge below. USPS rules then require 2 tabs along the bottom edge to seal the piece or else, you pay higher postage. The same thing can occur if the address is on the middle panel of a tri-fold and the piece is folded with the open edge down.
A better approach is to put the address panel at the top of both bi-fold and tri-fold pieces. The open edge is automatically up and the bottom 1/2 to 2/3 of the sheet is available for your message. It is often better than splitting up a tri-fold into message at the top, address in the middle, message at the bottom. Use 28/70 or 32/80 weight paper for the overall savings, better feel and higher opacity for two sided printing.
