yes, you could edit the shipping rules with eCart.
First, make sure the powerstore files are at the root of the Dreamweaver site.
then open the cart page and go to Insert -> WebAssist -> eCart -> eCart Object. In the object panel, click the edit button, then go to the shipping tab.
Create a new shipping rule for each price break, for example:
Taxable Total For the cart:
if the taxable tatla for the cart is "<" a value of "25"
Flat rate of 5