Try this change instead:
Code:
      // adjusted count for free shipping
      $item_total_count = $total_count - $_SESSION['cart']->free_shipping_items();
global $cart;
$chk_flowers = $_SESSION['cart']->in_cart_check('master_categories_id','14');
if ($chk_flowers > 0) {
  $chk_flowers = 15.00;
} else {
  $chk_flowers = 0;
}

      $this->quotes = array('id' => $this->code,
                            'module' => MODULE_SHIPPING_ITEM_TEXT_TITLE,
                            'methods' => array(array('id' => $this->code,
                                                     'title' => MODULE_SHIPPING_ITEM_TEXT_WAY,
                                                     'cost' => (MODULE_SHIPPING_ITEM_COST * $item_total_count) + MODULE_SHIPPING_ITEM_HANDLING + $chk_flowers)));