Priority Mail Not Calculating Properly

If I add 1 item to the woocommerce cart, the shipping cost would indicate $6.45, The weight of the item is about .25#. If I add a second item of the same type, the shipping charge is now $12.90 The shipping cost should still be $6.45 as the weight did NOT exceed 1#.

The only option I have selected is for Priority Mail. I have used my User ID as well as the default ID and the same thing happens. I have tried selecting and unselecting options and cannot get this to work. What am I doing wrong?

https://www.ads-software.com/plugins/woo-usps-simple-shipping/