Make custom field editable in customer shipment entry after invoiced

We require a custom field in our Customer Shipment entry form to track Freight costs. Is there any way to make the custom field editable\saveable after the shipment has been invoiced like the Tracking Number field?
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I’d say you could most likely place an event to detect when invoiced and then set the desired field to not be readonly - BUT - me thinks the BO itself may not allow changes to the record after it’s invoiced. I dont know that to be a fact, but you could test with BLTester on an incvoiced record

I will test thanks! The tracking number still saves but I suppose that is coded into the BO!

If you find that it does allow your custom field to be saved, you could maybe even handle this easy with a RowCondition and rule to enable the desired field