In the run up to Christmas I'd like to be able to offer an additional postage option along the lines of "Royal Mail Special Delivery" which would be £6 for one item plus a pound for each extra item.

Is it possible to add such an option with a little extra text saying that people who place their orders on or after 19th December should pick it if they want their items delivered in time for Christmas?

I currently have zone rates shipping set up for the regular shipping orders.

Any help gratefully received.

Thanks,
Ian