There are two things going on here. The first thing is that you can use the other shipping types like PARCEL but you will need to update your shipping file for USPS in the eCart folder, I'm attaching the file to this post. Make sure you put it in your WA_eCart folder.
The other thing going on here is that PRIORITY is a flat rate service, so the price will not change based on weight or location, it will changed based on the package type. Here is a link to a post that list the available options for this setting: