That sounds a little nuts, especially since you are planning a VBAC. Are they expecting that plan to completely fail or what? :/
I don't have any experience with VBAC/prepaying for CS, but I'm halfway and haven't gotten a single bill from my OB yet. The receptionist has definitely not reamed me out about money owed.. Sheesh. Even when there is an outstanding balance, they just send me a statement.