My car is nothing special. They would claim that prices change due to other people booking while you're going through the process, but I don't believe that.

I was booking well in advance - more than a month, and I was using the site after midnight.

There should be regulation against changing a price that is advertised, at least lock it in for a minimum of 5 or 10 minutes of selecting the fare. Bumping the fare up at the last step only a couple of minutes later is highly suspicious.