Change USPS package to envelope so that the dimensions are fixed, add to your theme functions.php file

gistfile1.txt

/**
 * USPS 4.0 introduced the ability for envelopes to be flexible you can disable this with this function
 * Simply set the type to 'envelope' instead of 'package' for any of the flat rate services that USPS lists as envelopes
 */

add_filter( 'usps_flat_rate_boxes', 'custom_usps_flat_rate_boxes' );
function custom_usps_flat_rate_boxes( $flat_rate_boxes ) {
    
    // Priority Mail Express Envelopes
    $flat_rate_boxes["d13"]["type"] = 'envelope';
    $flat_rate_boxes["d30"]["type"] = 'envelope';
    $flat_rate_boxes["d55"]["type"] = 'envelope';
    $flat_rate_boxes["d63"]["type"] = 'envelope';
    $flat_rate_boxes["d98"]["type"] = 'envelope';
    
    // Priority Mail Envelopes
    $flat_rate_boxes["d16"]["type"] = 'envelope';
    $flat_rate_boxes["d29"]["type"] = 'envelope';
    $flat_rate_boxes["d38"]["type"] = 'envelope';
    $flat_rate_boxes["d40"]["type"] = 'envelope';
    $flat_rate_boxes["d42"]["type"] = 'envelope';
    $flat_rate_boxes["d44"]["type"] = 'envelope';
    
    // International Priority Mail Express
    $flat_rate_boxes["i13"]["type"] = 'envelope';
    $flat_rate_boxes["130"]["type"] = 'envelope';
    
    // International Priority Mail
    $flat_rate_boxes["i8"]["type"]  = 'envelope';
    $flat_rate_boxes["129"]["type"] = 'envelope';

    return $flat_rate_boxes;

}