Shims for flm ultimate hybrid rear bulk

[Kigne]

Hi guys,
Just a stupid question about the fastlane hybrid bulks.
I don't understand how to fix the drive cup on the pinion gear.
The pignon gear is 5 mm in diameter where the drive cup goes, while the traxxas drive cup have a 6mm internal hole.
Do I have to use the small aluminum cylinder inside the kit? how I can fix it?
Do I have to buy 5mm drive cup?
Thanks a lot
and Merry Xmas
Marco