Commerce USPS Event Subcriber Example. Dimensional weights are automatically applied by the USPS Webtools API, but this file provides a solid example of how to manipluate the package array before a rate request is made. Make sure to register your event subscribers

DimensionalWeightSubscriber.php

<?php

/**
 *  Be sure to register this in your MYMODULE.services.yml file.
 *
 *  MYMODULE_dimensional_weigth.rate_subscriber:
 *   class: Drupal\MYMODULE\EventSubscriber\DimensionalWeightSubscriber
 *   tags:
 *    - { name: event_subscriber }
 */

namespace Drupal\commerce_usps\EventSubscriber;

use Drupal\commerce_usps\Event\USPSEvents;
use Drupal\commerce_usps\Event\USPSRateRequestEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Class DimensionalWeightSubscriber.
 *
 * @package Drupal\commerce_usps\EventSubscriber
 */
class DimensionalWeightSubscriber implements EventSubscriberInterface {

  /**
   * The minimum volume to trip dimensional weight.
   */
  const DIMENSIONAL_WEIGHT_MIN_VOLUME = 1725;

  /**
   * The dimensional weight divisor.
   */
  const DIMENSIONAL_WEIGHT_DIVISOR = 166;

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      USPSEvents::BEFORE_RATE_REQUEST => ['dimensionalWeight'],
    ];
  }

  /**
   * Alter the rate for dimensional rate requests.
   *
   * @param \Drupal\commerce_usps\Event\USPSRateRequestEvent $event
   *   The rate event.
   */
  public function dimensionalWeight(USPSRateRequestEvent $event) {
    /** @var \Drupal\commerce_usps\USPS\USPSRate $rate_request */
    $rate_request = $event->getRateRequest();

    foreach ($rate_request->getPackages() as $delta => $package) {
      $volume = ($package['Length'] * $package['Width'] * $package['Height']);
      if ($volume > self::DIMENSIONAL_WEIGHT_MIN_VOLUME) {
        $dimensional_weight = ceil($volume / self::DIMENSIONAL_WEIGHT_DIVISOR);
        $rate_request->setPackageValue($delta, 'Pounds', $dimensional_weight);
        $rate_request->setPackageValue($delta, 'Ounces', 0);
      }
    }

    $event->setRateRequest($rate_request);
  }

}