Hey guys,
I see several people here looking for how to make the Officejet 8600 print #9 envelopes which is not an option in the main printer driver for this model. However, this model printer uses PCL3 GUI for the printing language. This is the same language that the Deskjet 9800 driver from HP uses and it has these features you are mentioning. If it does not have the specific # envelope option you need it does have the ability to set up Custom Paper sizes in the print driver as well.
For information on how to use an alternate driver please see the appropriate link below for you:
The Windows option mentions the 990c or 450c drivers, however for the options you are looking for make sure to use the HP Deskjet 9800 driver.
Let me know how it goes,
Jason