[SOLVED] Intermittent Shipping Price

[beginner]

I am having an unusual problem with the shipping cost on a site I am currently developing.
I have set up the following price based delivery charge £0-£200 = £30, £200.01-£700=£45, £700.01-£100,000=Free.
However, when you add some items to the cart they show the shipping charge, while other items that are below £700 show the shipping charge as £0.

Here is a typical page that demonstrates the problem: http://www.tharlaproofing2.co.uk/category.php?id_category=17
Of the nine items on the page 4 will include the shipping cost and 5 show it as £0. I cannot see a pattern to this.

If anyone has a minute to take a look and let me know if they have any ideas or theories as to why this happens I would be extremely grateful.

Many thanks

[D-fox]

There's a module to simulate shipping costs see prestatools.com.
Asides that in the basket only default transporter's price is shown.

[beginner]

At last I've managed to figure it out. Although I based the delivery charge on cost of item, I left the default 'Free shipping starts at:' setting at 20kg. The entries that were showing £0 shipping had a weight above 20kg. I hope nobody went to any touble over this.