On Costco’s last earnings call, executives were grilled about a problem few companies have: how is the company managing crowded stores and jammed-up parking lots?

That dilemma is a sign of the times for membership-based warehouse clubs. More Americans have literally joined the club — fueling growth for Costco, Walmart-owned Sam’s Club and BJ’s.

All three retailers are opening more locations across the country. Shares of the companies have shot up in the past five years, with Costco’s stock up about 215% and BJ’s up about 305% since the day the Covid pandemic began in March 2020. And Gen Z and millennial shoppers have helped fuel the club channel’s gains, as trendier brands and more convenient digital offerings attract younger shoppers.

High inflation “brought the club channel more and more into focus,” said Bobby Griffin, a consumer analyst at equity research firm Raymond James. The clubs have long been known as a place to buy cheaper gas or bulk packs of household staples for less.

Yet the companies have continued to “up the ante,” he said. Merchandise has gotten sharper, private label offerings have become stronger and the shopping experience has gotten more enjoyable as the retailers have spruced up stores and added more technology, he said.

Clubs have benefitted from an element of surprise, too. Along with selling bulk packs of paper towels and Keurig coffee pods, clubs have caught the attention of shoppers with items that tap into a desire for dupes or go viral on social media — such as Costco’s gold bars, which racked up more than $100 million in sales in a single quarter.

Along with the breakaway hit of gold bars, the many card-carrying members of Costco prompted an unusual message from the U.S. Transportation Security Administration this year. As the government agency phased in stricter requirements for ID cards, it announced across its social media accounts in June that Costco’s membership cards don’t count as a Real ID.

And Costco’s loyal fan following helped it to post strong sales — and attract support — despite some backlash for sticking by its diversity, equity and inclusion policies.

As they pick up newer and younger members, the warehouse clubs see more room for growth.

Sam’s Club earlier this year announced plans to open 15 clubs per year going forward, along with renovating its approximately 600 current clubs. It’s expanding its footprint again after shutting 63 locations across the country in 2018.

BJ’s plans to open 25 to 30 new clubs over the next two fiscal years. The smallest club player, which has historically had more locations on the East Coast, has broken into new markets like Texas by opening four locations in the Dallas-Fort Worth area.

And Costco has stuck with an aggressive expansion plan of opening about 30 clubs per year, with just over half of those in the U.S. and the rest in other parts of the globe, CFO Gary Millerchip told CNBC. In early August, Costco opened four clubs in three different countries: Quebec, Canada; North Guadalajara, Mexico; The Villages in Florida and Richland, Washington.

Costco is opening some of its new locations this year in existing markets where its clubs are crowded, Millerchip said.

Club retailers still face pressure, though, including an uncertain job market and tariffs. The companies have laid out strategies to reduce their hit from the duties: Costco leaders, for example, said on an earnings call that they diverted imported merchandise with high tariffs to their warehouse clubs in other parts of the world instead of the U.S.

Clubs’ rotating brands and treasure hunt approach could reduce their vulnerability to tariffs. While the retailers sell imported merchandise like furniture and clothing, the bulk of sales come from groceries, and the retailers could swap out or drop an item hit by high tariffs, Griffin said.

BJ’s will carry more holiday items this year from the U.S. or countries with lower tariff exposure, said Bill Werner, BJ’s executive vice president of strategy and development.

As retailers digest the long-term effects of tariffs, Costco will give the latest read on its business, and the club channel, when it reports earnings on Thursday.

Sam’s Club, in particular, has used tech to stand out from competitors. Customers can skip the checkout line by using Scan & Go, a feature in the retailer’s app that allows shoppers to ring up their own items while browsing the aisles. About 40% of its transactions are through Scan & Go, Sears said.

It’s leaned on other tech, too, including automated floor scrubbers that free up employees’ time to help customers and high-tech archways at the exits that verify most purchases automatically instead of requiring an employee to manually check a receipt.

BJ’s, too, has capitalized on speedier digital options that appeal to busy families and younger shoppers. E-commerce sales at the club jumped 34% in the most recent quarter compared to the year-ago period. CEO Bob Eddy described the digital gains as a “generational unlock” that’s attracted busy families and younger shoppers.

Digital offerings have become a popular and lucrative part of BJ’s business, Werner said. Same-day delivery orders tend to be about 25% to 30% bigger baskets than in-club shops, he said. It charges a $15 fee for the deliveries, or members can pay $100 per year for unlimited same-day deliveries.

Still, Werner said BJ’s biggest selling point “comes back to value” with its pledge to undercut typical grocery store prices by roughly 25%. Food and household essentials like laundry detergent drive about 85% of its sales, he said.

At Costco, the largest club player by size and stock price, have more than doubled over the last decade. Yet digital sales, while growing, account for only a small part of its overall sales.

About 8% of Costco’s business comes from e-commerce, excluding third-party deliveries from Instacart and travel bookings, CEO Ron Vachris said on the company’s May earnings call.

Over half of members have downloaded Costco’s app, but its digital business is still in the early stages, Millerchip said on the company’s earnings call. He said as Costco will keep adding customer-friendly features, such as making it easier to search for items or save a credit card to speed up checkout.

“We still see it [e-commerce] as an area where we’d expect to outpace our overall growth,” he said on its earnings call.

The improved merchandise at clubs has caught the attention and ire of competitors as well. Lululemon filed a lawsuit against Costco in late June, alleging that the company violated patents by selling lower-priced dupes of its athleisurewear including hoodies, jackets and pants.

Costco’s CFO Millerchip declined to comment on the lawsuit.

Clubs have drawn both young and more established brands that want to get picked up by the retailers.

Wellness brand Frida, best known for popular baby supplies like the NoseFrida, is exploring how its products could be packaged and sold in a club, founder and CEO Chelsea Hirschhorn said. She said the club channel has become more appealing as it moves away from generic products and adds more modern brands.

For some members, including Patrick Bannon, club retailers’ eye-catching assortment can be a danger to the wallet. The 29-year-old graduate student joined Costco about two years ago. At least every other month, he drives to a nearby Costco for a shopping trip — even though the drive can take 45 minutes in traffic and on weekend and evening visits, it can be tricky to “move your cart more than an inch without running into somebody.”

For Bannon, it takes creativity to squeeze bulk purchases into the cabinets, freezer and fridge space of the one-bedroom apartment in the Boston area that he rents with his girlfriend.

In his apartment, he has currently stashed away five different types of protein bars, two pounds of frozen vegetables, two one-gallon jugs of vegetable oil and three or four pounds of frozen chicken.

He signed up for Costco to buy cheaper groceries and staples like trash bags, but he’s wound up purchasing giant bags of snacks, a new brand of cold brew coffee and even khaki pants.

“You get to be a kid in the candy store again,” he said. “Except it’s not all candy.”