Had another thought about packing weight.
I set up some checkout attributes, for packaging:-
standard (+100g) preselected
medium box (+500g)
large box (+1000g)
and this is cool, as it will always select the stadard option (+100 grams) and add it to the cart weight before checking which offline shipping rate to use. i check this was the case by altering the additonal weight for the attribute! all good so far.
How can I now, "automatically" select another one of these checkout atts, if the total of all the products necessitates this, ie if cart products are over 10kg then select large box checkout attr.