That is correct.
Regional Priority rate some times can be cheaper than flat rate.
Flat rate takes into account any weight up to 70lbs any distance in USA.
Regional uses exact weight and dimesions and distance in the calculation.
I shipped Regional flat 9 LBS within state ant was only $8. Same package priority mail $13 or even Standard post $9.
Make a box that fits your exact package size and regional may be cheaper!
You're also paying for the convenience and USPS providing you with a free shipping materials (box, label and no-tape required)>
You used to be able to take the free Flat Rate boxes and turn them inside out and re-tape them together with packing tape.. but they now print on the inside of them. One way around it is to wrap them in brown paper.