It is, and it isn’t.
The actual postage is usually but not always less, but there is much more “handling” involved. Priority and Express Mail packages can be prepared-on line; parcel post shipments must be prepared manually. But usually of more significance, Priority and Express Mail packages can be picked up by our mail carrier. Parcel Post packages must be taken to the Post Office, where someone must physically hand them to a window clerk. In our case, it’s a ten minute drive and a twenty minute wait.
So we could offer parcel post service, but the handling component of the shipping charge would be higher, making the total shipping cost equal to or greater than the cost of a priority service.