I just prepared a package that weighed 6 pounds 7 oz. I was so surprised to see that it could ship for $11.88 parcel post with USPS!
That is only $1 more than the medium flat rate box and less than the large box...that costs around $15 I think. (my things would not have fit in either of those)
My point is that we are being influenced to use these flat rate boxes and chances are we can do better. I sell fabric on here sometimes and I am always trying to get as much as I can in a flat rate box or envelope, but now I will be checking out parcel post again! Yes, it will take a little longer, but everything goes by air now, so it can't be that much different!