modifier for a "new price" in the level "order"

Hi at all!

I'm looking for an pricing solution for this business case:

Particular customers have negotiate fixed prices for every order. This fixed price apply if the volume of orders is within a specific range.

For example: the range is 300-1000$ and the fixed price is 500$

The customer pays for every order within this range 500 $

I can't find a modifier for this solution.

Thank in advance!

Regards

CFE