We’ve had great luck using Costco to book our rental vehicles. Easy to rebook if prices change. For what it’s worth.
That’s all well and good when it goes smoothly. Let me throw this out, you’ve booked a car through Costco. You show up and CarRental says sorry, we don’t have a car booked for you. Who do you want to deal with? The person standing right in front of you or Costco on a phone call? Because I guarantee CarRental will say “ you didn’t reserve through us”, go deal with Costco as they made the mistake.
I will admit to being overly cautious, partly because I travel ALOT. I’ve had many different things go wonky, both in the US and overseas. I’ve been burned by secondary booking twice and I’m now smart enough to avoid a third. It’s difficult enough trying to deal with one place, make it TWO, with competing agendas and it’s a nightmare as F4Champs posted.