I've been using PayPal Multi-order shipping. It's available as a link from your account details page (in PayPal). Once you figure it out and get it set up, it actually makes things quite a bit easier than using the standard PayPal shipping method.
The only things I don't like - it doesn't hide postage cost on the label by default (you have to do it manually), and you can't print the standard style PayPal packing slips automatically after printing your postage.