I use flat rate as much as possible, but the heads are too large for that box, it's only 3" tall. It was $39 to ship both of them in one box to VT, plus insurance using FedEX.
That's not a bad price.
I've been checking out the flat rate deal. If you can get it in the box and close it, without altering the size of the box, they will ship it. Flat rate is the shipping cost not the box shape.
The USPS box is bigger than the fedex flat rate box at 23 11/16 x 11 3/4 x 3 1/4. It will hold a m1 dual plane intake.