Thank you WebSource 5, that gave me the info I needed. I got it working. Here is my final source:
$products = $cart->get_products();
for ($i=0, $n=sizeof($products); $i<$n; $i++) {
if ($products[$i]['name']=="Ultra Race Fuel Concentrate") {
$this->quotes = array('id' => $this->code,
'module' => MODULE_SHIPPING_SALARGEPARCEL_TEXT_TITLE,
'methods' => array(array('title' => $this->quotes['error'])));
$this->quotes['error'] = 'NF Ultra Race Fuel Concentrate cannot be shipped by airmail.';
return $this->quotes;
}
}