In the file:
cart slash extensions slash default_usps slash storefront slash model slash extension slash default_usps.php
The following line is incorrect:
// Calculate girth based on usps calculation
$xml .= ' <Girth>' . (round(((float)$this->config->get('default_usps_length') + (float)$this->config->get('default_usps_width') * 2 + (float)$this->config->get('default_usps_height') * 2), 1)) . '</Girth>';
Girth is 2*width+2*height. USPS does use length+girth often, but that is not the same thing. Also that formula only works if length is longer than width and height, otherwise it needs to sort the longest side out of the formula.