The site "Pirate Ship" can be useful in obtaining discounted shipping rates. I think the PM Small Flat Rate box that is $9.15 at the Post Office is $8.40 there at present.
Yes, my mistyping... on both sides! $9.45 at USPS, $8.10 at Pirate Ship. They use USPS "Commercial" pricing, not usually available to mere mortals.