Wherre is the cart object found?
We currently use shipping costs based on weights, its in an excel file. The cost of shipping say for a package that weighs 500gram would lookup from the table below which would then return £2.50 as it is over 250gram but under 750gram.
Weight upto Cost
.250kg 1.99
.750kg 2.50
1kg 3.50
etc etc
thanks for your help