Let me tell you a little secret, there is no such thing as free shipping.
What happens here is the shopping cart which suppliers use does not allow you to add shipping costs for each individual item. So what most tend to do is add a price say £4.99 for anything under £30 which the shopping carts will allow you. Then orders over £30 they factor in the shipping costs into the final price.
Shipping costs include courier costs, packaging costs (boxes, labels, etc..)
What most couriers do is give you a price of say £9.00 to deliver anything upto 20kg. They don't charge less if it less than 20kg, the price is fixed.
Couriers are now being cheeky in adding fuel surcharges. That's the way it works, hope you have a better understanding.