USPS Flat Rate Shipping Boxes (South Parker)

It's an unopened package of USPS flat rate shipping boxes. I can see (4) 11 7/8 x 3 3/8 x 13 5/8 boxes. There are smaller boxes inside as well, but I can't see how many or what size. I think these would be handy for anyone shipping small-ish things out of their home.