Hi Ian,
Which version of the Cart and DNN are you currently running?
The Cart currently supports a feature called Shipping by zip code/postal code which allows you to configure shipping costs in the following ways:
Fixed shipping costs by country and “ship to” zip code/postal code.
Fixed shipping costs by shipping method, country and “ship to” zip code/postal code.
Fixed shipping costs by product category, country and “ship to” zip code/postal code.
You can find more info on this shipping method and configurations instructions on Pg. 189 of the SmithCart User's Guide -
Click Here
Please let me know if you have any questions.
Thanks,