I've decided to come at this sideways. I created a spreadsheet with all my invoices from the last year. I figured shipping cost per ounce using total shipping weight. Then I averaged the shipping costs, which works out to be about $.12 an ounce. I wrote formulas instead of breaking out the calculator, so I can update it every time with each new invoice. Which means for every formula I make, I can just multiply the total batch weight by the current average. Woot! I can also chart it and easily see if my costs are going up from specific suppliers or at certain times of the year. Double woot! If anyone has Excel 2007 (I don't know if it's backwards compatible) and would like a copy of the spreadsheet, send me a pm.
PS Seeing my total shipping cost for the last year also nearly gave me a heart attack!