When processing orders you might come across the "6003 The COMMERCIAL_INVOICE requires an electronic SIGNATURE" error.
This error is shown when the ETD (Electronic Trade Documents) usage has not been enabled for your account.
To fix this issue you need to complete FedEx's requirements to use ETDs with your account as described here: https://www.fedex.com/en-us/electronic-trade-documents/enable.html
You cannot activate electronic trade documents without uploading the letterhead and signature file to Fedex ship manager.
Once done and approved by FedEx, this error message will not be displayed and the orders will be able to process correctly.