increasestorequantityaction.php

См. документацию.

<?php
 
namespace Bitrix\Catalog\Document\Action\Store;
 
use Bitrix\Catalog\Document\Action;
use Bitrix\Main\Result;

class IncreaseStoreQuantityAction  implements Action
{
    use BaseStoreQuantityAction;

    public function canExecute(): Result
    {
        return new Result();
    }

    protected function getProductAmountNew(): float
    {
        return $this->getStoreProductAmount() + $this->amount;
    }
}