Fighting a Vendor

After months, the port-a-john folks sent me a bill for another months rent of their port-a-john unit. I was infuriated and so first thing this morning, I called to speak to them about it. No one answered so I left an irrate message on their answering machine.

I basically told them that they've got their nerve to send me a bill for another month's rent, and that if any money is still outstanding, they're the ones that should be paying me a refund.
I said that they only came out and serviced the unit one time during my entire rental, and I had to complain twice (or maybe three times) to get them to come out. As far as I'm concerned, I paid for a service that I didn't get.

I told them this and that they should be prepared not to get paid for this extra month's rental, and that if they really have a question about it, to call me. I left them my work phone number.
Needless to say, I didn't receive any calls from them, and I was in the office most of the day.
Hopefully, they'll just write this thing off, but if they pursue it, I'm going to pursue a counter action and get written statements from the workers about how the unit wasn't serviced. There's no way I'm going to allow this company to rip me off.

This should be a warning to anyone building a house, that little things like this need to be documented, because what seems logical at the time, isn't so logical months later. I had no reason to believe months ago that I could possibly be billed for anything more, so to get that bill a couple of days ago was such a surprise.