How to Add Shipping Charge Based on Order Total with Additional Charges

I'm having difficulty setting up a custom shipping fee in Drupal Ecommerce. I can't make heads or tails of the Additional Charges settings, and I can't seem to find any meaningful documentation on how to configure them. Searching the forums and trying to wing it have likewise resulted in nothing.

What I need is a way to add a simple shipping charge depending on the total value of items ordered, $5 for orders less than $25, $10 for orders less than $50, $15 for orders less than $75, $20 for orders less than $100, and free for orders of $100 or more. Strangely, what references I could find (for older versions, perhaps?) talk about a total value of items filter, but no such filter is present in the drop-down.

Is this even possible with this module? In case it's useful, I'm using Ecommerce 6.x-4.0-rc9 on Drupal 6.14. Any help would be appreciated.