/**
 * Remove USPS Flat rate envelopes from the available options
 * Once added the customer will not see any rates for envelopes
 * Only Small, Medium, and Large Flat Rate boxes will be used
 */
add_filter( 'usps_flat_rate_boxes', 'custom_usps_flat_rate_boxes' );
function custom_usps_flat_rate_boxes( $flat_rate_boxes ) {

	unset($flat_rate_boxes["d42"]);

	return $flat_rate_boxes;

}