38 lbs calculates just fine if the maximum weight exceeds the USPS module is not even active and does not even show on the checkout page. Below are my settings for Shipping/Packaging:

Country of Origin United States
Postal Code 28211
Enter the Maximum Package Weight you will ship 150
Package Tare Small to Medium - added percentage:weight 0:0
Larger packages - added packaging percentage:weight 0:0
Shipping Delay 3
Display Number of Boxes and Weight Status 3
Order Free Shipping 0 Weight Status 0
Shipping Estimator Display Settings for Shopping Cart 1
Display Order Comments on Admin Invoice 1
Display Order Comments on Admin Packing Slip 1