According to this article, RevContent appends the following parameter: rc_uuid=<UUID>
Unless I am mistaken, for the External ID, you should enter "rc_uuid" based on the above:

You can then still use {externalid} in the postback. Just be sure to add your API key and remove the extra spacing in {payout}:
https://trends.revcontent.com/api/v1/conversion.php?api_key=&uuid={externalid}&amount={payout}
I also had a look at RC's documentation and wanted to confirm that this is the right set-up - thank you @Traffic Armor for helping out.
@sp33dr4ge88 - please let us know if all works fine and should you have any other questions, feel free to ping me directly or contact our
Thanks. Changing to the rc_uuid worked everything tracks in