Do you sell items in quantities more than 1?
Now, I copied the following DIRECTLY from my listing template in Auctiva just now...mind you, it doesn't look exact due to no "boxes" or whathaveyou...but this is under SHIPPING TOOLS..and when you choose FLAT RATE SHIPPING..you will have Domestic Shipping Options and International Shipping Options, BOTH of which offer the additional text box to add "Additional Item Cost"
Domestic Shipping Options -
Enter "0" in the Cost field to specify FREE shipping for domestic and international options.
First Domestic Service: Cost: $ Additional Item Cost: $
Add another service