Excess weight shipping
I have a column for weight in my shopping cart. This is activated if the item I am selling is over the weight allowed by the postal service. If it is over the weight, I put a '1' in that column field. What I would like to do is, when that column field is activated, the cost of shipping has an excess added. (ie if the basic postage is £3 for an item but that item is overweight then it has an extra £2 added to the shipping) How do I do that?