This isn't something I would recommend doing. Sticking to contracts online is just not a thing unless you're dealing with HIGHLY reputable members who have something to lose. I know this is said a lot with just about any transaction, but even more so when it comes to a payment plan.
You can, however, use a middleman to hold onto the account until he pays. But since it's a monthly long thing, I'm not sure who would take that up (if anyone).
I wouldn't do it. I highly suggest against it, unless they have a LOT of history here and have been here for quite some time.