Around collect rates not updated

This is the class I use to overwrite the USPS collectRates function:

<?php namespace MyCompany\AmUsps\Plugin;  use MyCompany\Shipping\Logger\Logger; use Magento\Quote\Model\Quote\Address\RateRequest; use Magento\Usps\Model\Carrier; use Magento\Shipping\Model\Rate\ResultFactory; use Magento\Quote\Model\Quote\Address\RateResult\MethodFactory;  class UspsPlugin {     protected $  _logger;     protected $  _rateResultFactory;     protected $  _resultMethodFactory;      public function __construct(         Logger $  logger,         ResultFactory $  rateResultFactory,         MethodFactory $  resultMethodFactory     ) {         $  this->_logger = $  logger;         $  this->_rateResultFactory = $  rateResultFactory;         $  this->_resultMethodFactory = $  resultMethodFactory;     }      public function aroundCollectRates(Carrier $  subject, Callable $  proceed, RateRequest $  request) {         $  this->_logger->info('-- Inside collectRates USPS');         $  result = $  this->_rateResultFactory->create();         $  method = $  this->_resultMethodFactory->create();         $  method->setCarrier('USPS');         $  method->setCarrierTitle('setCarrierTitle');         $  method->setMethod('USPS');         $  method->setMethodTitle('setMethodTitle');         $  method->setPrice(9.99);         $  method->setCost(9.99);         $  result->append($  method);         return $  proceed($  request);     }  } 

Everything looks fine since I’m not getting any errors, but the rate set in setPrice/setCost doesn’t show up in the Checkout Page. The price shown there is the one collected by the original USPS collectRates function. It seems to me that $ result->append($ method) is not doing its job.

What am I doing wrong?

Thanks a lot.