Use Customer account DFF in price adjustment matrix and sales order total

I am trying to use DFF define at customer account level (number).
This number should be comapred to sales order total amount.
if sales order total amount > customer account DFF value => update one specific item price in the sales order
Can't figure out how to setup this behavior.
The initial customer requirement is to nullify shipping charges (materialized by an item in the sale sorder) if the sale sorder amount is superior to an amount defined at customer level.

Any idea how to deal with that?

Thanks a lot