You can create your own flat rate shipping charge rules within eCart. You can also have it go to another page based off of some criteria like total weight in the cart.
An example of a flat rate shipping rule would be to go into the cart object and in the shipping tab add a new rule. For the criteria you would say based on the subtotal of a column and use the total weight column. You can then specify the weight amount that will trigger the flat rate shipping. The in the calculation portion you would just enter the amount you would like to charge.
The charge rules are very flexible and you can do many things with them to add to or take away from the total. There is more info on the eCart tool and how to configure discounts on the support page here:
Take a look in the solution recipes for one about the discounts, they work just like the charge rules.