Hi jomomma,
Thanks for posting about this behavior here. We have noticed that selecting USPS Parcel Post is causing the Dimensions fields to become greyed out when using calculated shipping and our engineering team is currently looking into what needs to be done to correct that problem.
If you have not done so already, please feel free to file a support case about this issue from the "Help" tab on the Auctiva site and we can keep you posted on the issue as additional information becomes available.
-Mike