Hello Tracey, and thank you for posting to our community!
I'm very sorry for the delay in response, as I mentioned in the email I sent to you, I have asked our management to look into this issue further.
In regards to Parcel Select shipments being cheaper than Priority mail, since both Parcel Select and Priority Mail package services are weight and destination based, this means the rates are calculated by the distance the package is traveling, in addition to it's weight.
You can view more information about Parcel Select here: What is Parcel Select Ground?
You can also view our Parcel Select Commercial Base rate table here: Parcel Select Ground - Commercial Base (Rate Table) and compare that to our Priority Mail - Commercial Base (Rate Table) here.
While the two rate tables are not identical, you will see that many of the rates are the same for both Parcel Select and Priority Mail package services. You may also notice that even when heavier, some Parcel Select rates are still cheaper when shipping to mid zones, like Zone 5.