では、usc-e-shop/classes/itemList.class.php の251?269行目を以下の記述に差し替えてください。
$query = $wpdb->prepare("SELECT mc.meta_value AS item_code, mn.meta_value AS item_name,
meta.meta_key AS sku_key, meta.meta_value AS sku_value, te.name AS category, post.post_status,
CASE post.post_status
WHEN 'publish' THEN '" . __('Published', 'usces') . "'
WHEN 'future' THEN '" . __('Scheduled', 'usces') . "'
WHEN 'draft' THEN '" . __('Draft', 'usces') . "'
WHEN 'pending' THEN '" . __('Pending Review', 'usces') . "'
WHEN 'trash' THEN '" . __('Trash', 'usces') . "'
ELSE '" . __('Closed', 'usces') . "'
END AS display_status,
post.post_type, post.post_mime_type, post.ID
FROM {$this->table} AS post
LEFT JOIN $wpdb->postmeta AS mc ON post.ID = mc.post_id AND mc.meta_key = 'itemCode'
LEFT JOIN $wpdb->postmeta AS mn ON post.ID = mn.post_id AND mn.meta_key = 'itemName'
LEFT JOIN $wpdb->postmeta AS meta ON post.ID = meta.post_id AND SUBSTRING(meta.meta_key, 1, 5) = %s
LEFT JOIN $wpdb->term_relationships AS tr ON tr.object_id = post.ID
LEFT JOIN $wpdb->term_taxonomy AS tt ON tt.term_taxonomy_id = tr.term_taxonomy_id
LEFT JOIN $wpdb->terms AS te ON te.term_id = tt.term_id ",
'isku_');