Sounds like a pretty simple simple quantity discount.
you will need to create a separate rule for each quantity break on the shipping tab of the eCart object, for example:
1 - 12:
Triggers:
"SubTotal for any column"
If the Subtotal for column "Quantity" is ">" a value of "0"
AND
"SubTotal for any column"
If the Subtotal for column "Quantity" is "<=" a value of "12"
Calculation:
Flat rate charge of "12"
the other rules will follow the same pattern, just changing the quantity and the flat rate charge.
