My post office has no problem with extra tape on Priority Mail flat rate card board mailers. As long as you do not put the tape over the words FLAT RATE. You see regular Priority mailers look the same as flat rate mailers. So if the you cover up falt rate it looks like a regular priorty mailer, and postage is incorrect. With 3-4 lbs you need a little extra tape to be sure of no rip's.