I have a problem getting a shipping class added at checkout. My default option for products is free shipping, but I want to add an additional £4.99 postage charge for specific products. So I added a shipping class, and assigned it to the product. But when I tested it, the extra shipping cost is not added to the cost of the product at checkout. Instead 2 radio buttons appear allowing the buyer to choose free shipping or the extra flat rate of £4.99. How do I get the £4.99 to be added automatically at checkout?