Why would anybody ever use this service? Why would I use a potentially 8 day service to save pennies? The fact that this service comes up as the default USPS service, is a giant pain and a huge chance for making a mistake. I use USPS priority for 95% of my orders. I would very much like to see an ability to just set that as the default. It's a big time waste to have to keep doing it manually on each order, especially with the lag as each change calls back to get a new price quote. Or at least just trash the select ground. I can't imagine anybody would use this.
I was of the opinion that USPS Parcel Select Ground is quite cheap as compared to a priority service. In fact, this is the reason many customers prefer this service over any other. Are you sure the difference comes out pennies?
Regarding making the USPS Priority service as a default, this can be easily achieved with the Multi Carrier Shipping label app. This app allows you to create automation rules that would enable automatic selection of any service as default.