Hi Craig,
There are some options for promotional shipping rates, but you must set them up on eBay first. I don't think you'll be able to set them up in the way you are wanting, but at least they give some choices
If the option isn't available on eBay, we obviously can't make it available here either.
Go to your My eBay page and click on your site preferences under the "Account" tab. You'll see a section called "Shipping Preferences" that you can open up and see the different options available.
Once you have set up a promotional shipping option, you will need to go to your Auctiva account and update the shipping discounts. You can find the button to update them by going to the My Account tab and selecting eBay> Settings. Then when you create a new listing, you can check the box in the shipping section that says "Apply my eBay promotional shipping rule" and they should be applied during the checkout process.
I hope that helps a bit