Shipment External ID 1 copy to Shipment Costs Document

I have noticed that if there is something in the External ID 1 (VTTK/EXTI1) on the Shipment Document, it gets copied to the Shipment Costs External ID 1 Field (VFKP/EXTI1) on the Shipment Costs document. If the External ID 1 field on the Shipment Document is blank, then the External ID 1 field on the Shipment Costs document is populated with the Shipment Number. Is there some config that I could set to only have it copy the Shipment Number all the time and not the External ID 1 data?

Thanks,

Russ