services:
  Customize\Service\PurchaseFlow\Processor\TaxProcessor:
    public: false
    autowire: true
    decorates: Eccube\Service\PurchaseFlow\Processor\TaxProcessor

<?php

namespace Customize\Service\PurchaseFlow\Processor;
...
class TaxProcessor extends \Eccube\Service\PurchaseFlow\Processor\TaxProcessor
{
    /**
     * @param ItemHolderInterface $itemHolder
     * @param PurchaseContext $context
     *
     * @throws \Doctrine\ORM\NoResultException
     */
    public function process(ItemHolderInterface $itemHolder, PurchaseContext $context)
    {
dump($itemHolder);    // ★呼ばれない
        ...
    }
}

$shoppingPurchaseFlow: '@eccube.purchase.flow.shopping'

    /**
     * @param PurchaseFlow $shoppingPurchaseFlow
     * @required
     */
    public function setPurchaseFlow(PurchaseFlow $shoppingPurchaseFlow)
    {
        $this->purchaseFlow = $shoppingPurchaseFlow;
    }