Cost Fixed

Problem: Limit Ability to Change Financial Data
Is there a way to lock the shipment amount from changing, after the voucher has been paid?
Action check only works for one shipment but when recalculating costs on multiple shipments the Action Check does not fire. Therefore I am looking for other options.

Proposed Solution: Automate Cost Fixed action. Is there a way to have the Cost Fixed box automatically checked if the voucher has been paid? Agent Logic..