This is not supported in power store 3.
when you say per box, is each item a box? or is a box a specific number of items?
Is fruit the only type of item that you will be selling?
or will this per box rate apply to only some items?
Power Store 4 will allow you to set different rates by region, but may not be able to handle the other aspects of what you are trying to accomplish.
You may not be able to do this using either version of power store, but would need to use eCart.
If you would like to hire someone to develop this for you, you can post a request in the job board.