There are a couple of ways that you can apply shipping in eCart. You can make use of any of the available shipping providers to get a shipping quote from their api based on the weight, size and destination of the package.
If you do not use the real time shipping quotes you would need to create your own shipping rules. If you did it this way you would need rules to account for all of the possible shipping scenarios and costs.
If you are able to use any of the shipping quote services I would suggest that you do it this way, otherwise you will need to make quite a few complex shipping charge rules to account for this.