Flat rate boxes are free from ur local post office so u can grab different sizes and see if it will fit in any, smaller the better, unless it only fits in the large one and they aren't very heavy. Large flat rate is like $16 so if u don't think it would cost that to ship them then use a regular box. u could also bring them to the post office and ask them if it would be better off in flat rate if it fit or not.