Tracking number not uploading to Amazon - R+L Carriers

Not exactly an API question, but someone might have came across it while making an API request.

On Orders section, we are having issues writing tracking numbers back to amazon for a carrier called R+L Carriers. When Ordoro attempts to write the tracking number it fails and in the log we see a message that says, “The carrier-code field contains an invalid value. To correct this error, choose from the valid set of values.”

If someone is familiar with this carrier, wondering what name do we use so amazon would accept it?
We have tried rnlo, R+L, RL Carriers.

Thank you,