Free shipping on Zero weight products?

Hi,

Is it possible to have Market Press set the shipping cost to zero if a product has a weight of zero?
My client wishes to ship products internationally, but he charges shipping on CD's but on printed magazines the shipping is included in the price. This is how he's always done it so doesn't wish to change his pricing structure.

I managed to fiddle it on products shipped to the same country by entering an extra shipping cost of -1.00 which removed the shipping cost locally, but it of course won't remove the shipping cost internationally.

I tried setting the weight of the products in question to 0.00kg and then everything else to 1.00kg but it still insists on charging shipping on everything.

Can I change it so shipping is charged on any order weighing 0.50kg or more? Anything under that is free?

Thanks in advance... otherwise I'm loving MarketPress!

Cheers

Justin