Sixt can be all over the place on prices. I rented from them two weeks in a row at DFW for good prices. A couple of weeks later they were twice the price of other companies and I've never found them cheap there again. Booked a month long rental for the same place and they were once again the cheapest. Costco is pretty consistently cheapest in the long run, particularly if you keep checking back for price reductions.
Sheila
As I stated, in my case Costco is the lowest price by a long shot. When I checked last week they went up $40. I'm glad I booked when I did.
I found a deal giving me $50 worth of coupons for joining Costco through MyPoints.
I figured I'd join rather than have a hassle over the rental
if they ask for a card. I upgraded to the upper level, was told if my rebate falls short of $55 to bring the check in and they will make a new one for $55.
The CC they use here is an American Express that has no annual fee and a cash percentage rebate depending where I spend.
This solves my original question.
My rental will be in Phoenix for 11 days starting on Jan 9th. With the NCAA football championship game on the 11th, I think cars will get scarce the closer we get.
Thank you all for your comments.