Shipping calculation in powerstore

Hello
I have Powerstore 2 and i live in Greece. I have almost finished an e-shop project for a client but there is something last left that i have to configure:

None of the shipping providers (FedEx, UPS) are supported in Greece so i cannot use either of them in order to make a shipping calculation. What i need to do is this:
I want to create a custom code script that calculates the total weight of the products in an order and apply shipping costs like the example below:

if total weight of products is less that 1 Kg then the shipping cost is 1 euro
If it is between 1kg and 5kg the shipping cost is 5euro
If it is more than five kg then the shipping cost is...

Please note that do not own eCart. I do not know if it has something to do with it.

I am a designer and not a developer, and my knowledge in coding is limited.

Therefore i have hired a developer to do this for me.

First of all i would like to ask you if what i mentioned above is possible. And if there is any help i can get from any of you it would be very much appreciated. Is there anything specific instructions i have to give to my developer?

Thanks in advance.
John