You should be able to use your own modem. I bought one for mine but have not ordered service yet as I need to get ready for the line to be run (trees cut back.)
On their application pages, it does ask if you have a modem.
As for the fee, they have been known to charge even if you own the modem. Hopefully not, we'll see.--
Embedded Systems Consultant,
SHDesigns home - DIY Welder