Inconsistent Shipping Charge

[paul in london]

We have a very simple shipping scheme on our website. For all orders of £200.00 and under, shipping is £20.00. For all orders above £200.00, shipping is free.

We are having some problems with this scheme however, we have been testing it from computers in our office after we noticed some errors with customer orders and it appears that in some cases shipping is applied, in some cases it isn't and in others £10.00 is added. This is based on ordering the same products from those different computers, so I wandered if this was a caching error somewhere along the line? The performance is equally inconsistent across IE 6, 7 and Firefox so I don't think it is browser related.

Has anyone experienced anything like this before?

[leehack]

Provide screenshots of your shipping tables and screenshots of the shipping tab on the products that are problematic, almost certain to see either a mistake in the settings or that you have overridden things on these products. You will probably never get as far to showing us, as in doing this you will probably see the error.

[paul in london]

Screenshots

Please find the screenshots attached. In Screenshot 3, the only scheme we have defined is the one that is selected. Please don't hesitate to ask for more shots if you need them!

[leehack]

And what's a problematic product and which country is being selected?

[paul in london]

(one) of the products causing problems is at

http://www.flooringsales.co.uk/acata...o_2260015.html

This product is the one featured in the screen shots. When it behaves correctly the shipping is added on the first page that is displayed after the user has clicked on the 'add to cart' button, before the user specifies the country they are in. Given the nature of the products we sell, there is little or no market outside the UK.