In short, the online quote from paylesscar.com is a scam. Payless at PHX doesn't honor it.
Longer story:
I am very grateful to other reviews who shared their experience with Payless at PHX. I visited Phoenix this September and needed to rent a larger vehicle for a group of four. Although I have never rented from Payless, I found their rate to be very competitive: their mid-size SUV price was similar to the mid-size sedan price from the second cheapest one (Enterprise in my case) in PHX.
So I made a reservation on Payless website, but then I found those negative reviews here. Being concerned, I made another reservation with Enterprise for the same rental period as a backup.
When I arrived at PHX, the Payless associate told me that although I reserved a mid-size SUV, they could only give me a compact car because all higher categories were not available, and they could not lower the price for only giving me a compact car.
Luckily I had a backup and immediately walked away from Payless. So for potential renters, please do yourself a favor and avoid Payless.