When combined with Pay Now, I've found that Costco Travel codes have the lowest rates of anything else I could find.

Every year I go on a camping trip for July 4th and end up renting somewhere between an Intermediate SUV to a Suburban depending on how many people are going. This year we're aiming for a minivan and the lowest price that I could find among any rental company (including Pay Now for other companies too) was Avis... $297 (before taxes and before a $35 off coupon) for a 5 day rental over the 4th weekend. Every other rental company was quoting me at least $569 (before taxes) for the same vehicle class, even with Costco discounts for Budget, Alamo, and Enterprise.

I must have bad luck with Expedia/Hotwire/etc. They always consistently give me the worst rates. This year, they were quoting me up to triple what I was paying with Avis.

Each year is different. Last year Alamo had the cheapest rates. The year before it was Enterprise. I always look for other codes to use to find the cheapest rates (and I compare prices for about a month or so, letting it get as close to the travel date as I possibly dare, worrying that they'll be sold out of the larger car classes for the holiday), and Costco always comes out ahead. When I book, I always book for the lowest class of car/size I'll accept, then ask on the day what options they have that I can upgrade to and see if their additional price per day is acceptable. Sometimes there's a free upgrade but I don't always count on it.

Last year, with Alamo I reserved a Prius and grossly underestimated how many people (and gear) were going camping, but they upgraded me to a large SUV for maybe 1/6th of the price of what they were advertising online when I originally booked. The Prius was about $150/week, they added about $10/day to upgrade to the SUV, even though the car size problem was my problem and not theirs. I think at the time the large SUV was listed at something like $1000/week online. Loved it for $220.

Apparently I can't use my Wizard number or preferred when I use the pay now option with Avis (or it might be the Costco AWD), but it's not really worth paying double over when I only rent 1-3 times per year.

I rented an Intermediate sized car from Avis today for three days (reserved on Tuesday) for $13.83/day with pay now under Costco's codes, hoping for a Cruze. When I got to the counter, all they offered was an HHR (didn't see any Cruzes on the lot). Upgraded to a full size for $17.83/day. Upgrading at the counter was still even cheaper than purchasing a fullsize outright with the Costco codes and pay now ($19.35/day), so I'm pretty satisfied.

Use at your own risk if you aren't a Costco member! Sometimes they ask for your membership card, sometimes they don't:

http://www.costcotravel.com/#12_rentalCars_avis