MarketPress Shipping Glitch

I need a flat-rate shipping applied to all products, except for digital goods in the store. To do this I've used the weight table, and set all physical goods to 1 lb and the $ amount, and all digital goods to 0 lb. Pretty close but...

This is the glitch: if I add a mixture of products (digital and physical) to my cart, then upon viewing my cart remove all physical products - the S&H charge still remains. Is it not re-calculated upon updating the cart? I don't want to charge people shipping for something that's delivered digitally.

Thanks for any guidance, or suggestions on better ways to approach this!!