I need to add Unlimited Data without the xfi modem to my account

I can find the option to add unlimited data to my account for $25 if I also have an xfi gateway shipped to me for self install.  I am happy with my current modem and mesh network in my house so I would like to only add the unlimited data for $30 per month.  

I know this may sound dumb to spend more to not have the xfi gateway, however I dont want to waste the shipping cost up front and dont want to hastle with my account down the road if possible.

Kind of the Ron Popeil train of thought.... "set it and forget it"