Have you ever been able to get these to run successfully on this device in the past?
You did not specify what the jam code is (which may narrow the cause), so I will provide some general info below.
Envelopes on this device can be tricky. On US C70 products, the machine requires a special Envelope Fuser. Without the Envelope Fuser, the machine will not provide Envelope Attributes when loading them in the tray (this can cause issues). Once the Envelope Fuser is installed, the machine will provide envelope attributes when loading the envelopes into the tray. When you are done running envelope jobs, reinstall the normal fuser to run non-envelope jobs. Otherwise you will get an error when trying to run normal paper using an Envelope Fuser.
The machine may require an Envelope Fuser in order to run the A7 envelopes successfully. Also, only load envelopes in the Bypass Tray (no other tray). And the orientation when loading into the tray makes a difference as well. And depending on the thickness of the envelopes, they may jam when entering the finisher (due to steep upward turn). You also want to be sure that Face-Up Printing is selected in the finisher settings in the Fiery properties for the job.
Jams can also occur if you try to fool the machine attribute settings (when loading the envelopes).
Again, envelopes are very tricky on this device.
We may be able to help further if you provide more details.
Sounds like you have tried everything already, so it would be a hardware escalation: 1-800-821-2797 or 1-800-835-6100.
I have been trying to print addresses on A7 envelopes for about an hour now. Despite that I follow all the guides (heavyweight 2, tray 5, etc), the envelopes still continue to get jammed. I have tried printing with the flap open, the flap closed, the flap tucked into the envelope itself, and other things. I cannot get this envelope to finish. Thank you so much for any help you can provide. It's greatly appreciated!