Well, I disabled the variable price because it does not allow the use of the buy it now button, which is one of the main points of what I am trying to do. Anyhow, I'll fiddle with it a bit more.
As for people paying less than they owe, that's not a concern, because this is just a "Payment on Account". If they fail to pay within their payment terms, then regular collections actions will ensue.
I do like the nice, easy e-mail links. I'll keep that in mind. Eventually we will be moving to a solution that will interact directly with our accounting software via SOAP, and actually post the payments to a customer's account, but I need a professional to do that.