Ok...our builder's sales rep called and was mystified. A subcontractor sent you a bill? Yes.
According to Felicia, this particular subcontractor sent the invoice to the builder only recently (remember folks, we closed on our house in October) - apparently the builder also laughed at them.
As a last ditch effort, they apparently sent us a bill hoping we'd just pay...hah! Like I'm going to send some doofus $700 simply because I got a bill and not investigate it!
I've also decided that since we could not get them to call us back when we were trying to schedule the work, that I'm not going to call them about the invoice they sent. When they call me, I'll say something like..."do you find it annoying when people don't call you back in a timely matter? Funny, I find it annoying too!"....