M5200n will not take custom envelope size from envelope feeder.

I have an M5200n with an envelope feeder.  Size 10 envelopes work fine through the envelope feeder.  But now trying to print to a custom envelope size the printer will always asks for the envelope from the manual paper feed vs the envelope feeder.  I have the printer configured as such:

 

Printer Settings:PAPER SIZE:Env Feeder Size = Other Envelope

 

I created a custom form in the driver with the correct dimensions of the custom envelope(It's "8.25x5.875").

 

I can't use the manual paper feed, because there are 500 envelopes to print and the manual paper feed creases the envelope all to heck.

 

Source document is a mail merge word Document.

 

Thanks for any ideas you can provide.