A paper doesn't define an output tray, job types do.
If printing, your best bet is to install the driver 2 times, but change the default/preferences and the name of one to Envelopes with the center tray, then use that when doing the envelope job
How can I set the output tray on a WC7970 for envelopes? Settings allow for selecting output for fax, copy and print, but not envelopes. I would like envelopes to output to center tray vs left side tray. Both tray 5 9x12 envelopes and tray 1 #10 envelopes default to side tray. This is a replacement WC7970 and our original system outputed envelopes to the center tray.