Спасибо огромное! Сделал как написано в этой инструкции.
Единственное, не работал код для изменения mailalert.php, в функции "hookActionValidateOrder" :
foreach ($products as $key => $product)
{
$manufacturer = new Manufacturer($product['id_manufacturer'], $id_lang);
$items_table .=
'<tr style="background-color:'.($key % 2 ? '#DDE2E6' : '#EBECEE').';">
<td style="padding:0.6em 0.4em;">'.$product['product_reference'].'</td>
<td style="padding:0.6em 0.4em;">'.$manufacturer->name.'</td>
<td style="padding:0.6em 0.4em;">
<strong><a href="'.$url.'">'.$product['product_name'].'</a>'
.(isset($product['attributes_small']) ? ' '.$product['attributes_small'] : '')
.(!empty($customization_text) ? '<br />'.$customization_text : '')
.'</strong>
</td>
<td style="padding:0.6em 0.4em; text-align:right;">'.Tools::displayPrice($unit_price, $currency, false).'</td>
<td style="padding:0.6em 0.4em; text-align:center;">'.(int)$product['product_quantity'].'</td>
<td style="padding:0.6em 0.4em; text-align:right;">'
.Tools::displayPrice(($unit_price * $product['product_quantity']), $currency, false)
.'</td>
</tr>';
Заменил
$manufacturer = new Manufacturer($product['id_manufacturer'], $id_lang); на $manufacturer = new Manufacturer((int)$product['id_manufacturer'])
;
и все заработало.