I booked my rental car through Costco Travel around the end of June and it never prompted me to enter my CC info. I made multiple ressies across multiple days (had not finalized plans yet) for this reason since no payment of any kind was required to make a ressie; just cancelled them as we began to finalize plans. Granted, ours was through Enterprise and not
Avis, but one of the ressies was through Avis as well and no payment required at the time of booking. When we got to Enterprise, I simply gave them my name and reservation number from Costco and then handed over my CSR. Nothing more, nothing less. I signed up for Enterprise's loyalty program just to have one since the rental would count for something I figured. But as
@calypso726 said, decline any insurance through the rental agency. Using the CSR will suffice.