I think I found a solution --

In my Admin - Configurations - Shipping/Packaging, the line Order Free Shipping 0 Weight Status was set to 0 (No). I changed it to a 1 (yes) so that if there is no weight to the order, the order has free shipping.

Since my products at the moment are downloadable, they have a 0 weight. I experimented with it, and it seems to work. Now, when I have both free and for purchase items in the cart, the shipping is still free. I also tried changing the weight of a product, and then shipping gets charged.

I think it is good. I hope I haven't messed up anything by changing that config line!! <fingers crossed!!>

Katie