thats not true. take an item on a regular box and tell its gonna be ship priority and stills be the same, on some case less than using flat rate box.. the only might change is if it weight more than 70 lbs. meanwhile does not matter too much if u use flat rate box or not when using priority service. example from your zip to mine.. 19.75 using flat rate and 31 using not flat rate 12x12x6. is more, yes, but no double not 3 or 4 times.