Thanks for the replies. I have an MBNA Platinum card and called, and they actually require that the rental be made directly through the rental agency. So in my case, I saved $20 by bidding on priceline for a 2-day rental, but if I get added insurance through autopac, this will be an additional $50, so priceline buyer beware.
My follow-up question is: what is the most affordable (and trustworthy) way to get required insurance for a priceline car rental in US?