A rental-car booking trick that has quietly existed since 2000 is having a moment on TikTok and Reddit: renting your car through Costco.

Costco Travel, the warehouse club's in-house booking arm, lets members compare rates from the big rental companies — Alamo, Enterprise, Avis and Budget — with eligible discounts and coupons applied automatically. The rental firm itself still handles payment, insurance, toll transponders and the actual car; Costco just gets you a better price at the front door.

The standout perk is the free additional driver. Participating rental companies normally charge a daily fee for a second driver, but Costco bookings include one at no cost — and the extra driver doesn't need to be a Costco member. Only the person making the booking does, and membership starts at $65 a year. Eligible reservations can also be cancelled without a fee (prepaid rates may differ), and Executive members earn the store's 2 percent annual reward on qualifying travel purchases.

Does it actually add up? One Executive member, quoted by The Washington Post, booked a compact SUV in Los Angeles for $573 including taxes and fees for a 10-day trip — about $88 cheaper than going direct to Alamo — and skipped the $12 to $15 daily second-driver charge on top. With the 2 percent reward factored in, a single rental nearly covered her $130 Executive membership.

Travel experts put typical savings at 10 to 25 percent, with occasional discounts of up to 40 percent. But the deal isn't automatic: rates swing widely by location, dates and vehicle class, and NerdWallet's Sally French points out that credit-card perks or other promotions can still beat Costco's price. Her advice, broadly shared: compare the full total — taxes, fees, extras — rather than the headline daily rate.

One detail worth noting: Costco Travel negotiates its own rates in-house with the rental companies, while rival warehouse clubs Sam's Club and BJ's rely on white-label third-party agencies. That, travel writers suggest, is why even members of those rival clubs often find Costco's car rates the sharpest.

For anyone renting a car for a week or more — road trips, family holidays where a second driver is essential — the numbers are hard to ignore. If you already hold a Costco membership, checking Costco Travel before booking direct should take a couple of minutes and could save real money. If you don't, the maths only works if you'll use the membership elsewhere; the $65 base fee can pay for itself in grocery savings, but one cheap rental alone probably won't cover it.

The sensible play: price your rental both ways, including every fee, and factor in whether a second driver is included. If Costco wins, cancel-eligible bookings mean you can keep shopping around right up to pickup.