UPDATE:

I just called Avis and they adjusted my bill for a $164 credit. Close enough to the $174 I had calculated, and no need to dispute it with the credit card people. That's a last resort, FCTSTY.

I believe the problem stemmed from my original reservation of four weeks, which they automatically convert to one month. The lady on the phone said that their monthly rate begins at 28 days.

If I run into this situation again, I will make a reservation for three weeks and that should eliminate the problem of new rates being pulled out of nowhere.

[This message has been edited by JS (edited 03-10-2003).]