I just posted a shipping option in the MODx forums (http://modxcms.com/forums/index.php/topic,15055.msg100098.html#msg100098). The text is below.
One shipping configuration I've used and love is a per product pricing such as:
What this does for shipping costs is:
$3.00 - First item
$2.50 - next 2 items
$2.00 - each additional item after that
One problem I ran into was for multiple products. The shipping is added for each product and can be too expensive. So a modification I made to this for one store was:
Get the total shipping for all products, then make the final shipping the most expensive.
I have the code for this if you'd like to see it.