First you need to create the new media type for the device in the Media and Costing Tab. More info on how this is done can be found on our Adding, Changing and Deleting Media article.
Then in the Devices and Terminal go to the Device group you want to add the additional media type to, right-click it and go to properties.
Then from there you click the additional media button and in that small window check the media type you want to add and hit okay.
Now the changes wont take affect immediately so if you're looking to roll this out asap we recommend giving the Argos Communication service on the server a restart.
Then when the user goes to bill their job they just change the media type from a little dropdown under the device name in the billing client.