Getting job to output to stapler main tray on C8000?

[AB1984]

When printing directly to our C8000 (from printshop mail, if it makes a difference) I set the output tray to 'Stapler main tray', but it always get output to the sub tray. Does anyone know why this is and how I could rectify it? either from the driver, or changing the output settings mid-job on the touch screen hopefully.

Any help appreciated,

Cheers!