Unfortunately you can't regulate the rate or frequency at which mail is sent out by shopping carts. What you may want to do is to install an additional piece of software like phpLists. It's a bit clunky to use, but it does allow you to stagger any mail-outs e.g. set it to send 10 emails, stop for 30 seconds, then send 10 more. This is perfectly acceptable to most hosts. You can add a link to it on your homepage and get people to subscribe to your Newsletter in that way.

Vger