But the warehouse operator, a unit of Wal-Mart Stores (WMT), has been looking to bridge that gap by strengthening one area where Sam’s Club has an edge over its arch-rival: the integration of its digital business and its 655 or so stores, which the retailer refers to as “clubs.”

Case in point: last year, Sam’s completely overhauled its in-store pick-up service for online orders. As a result, use of “Club Pick-Up,” as the service is called, rose 46% year-over-year during the holiday quarter, a bright spot in an otherwise difficult season for Sam’s Club that saw comparable sales fall 0.5%. (Costco has beaten Sam’s Club on that front for seven years running.)

Club Pick-Up lets customers build and save shopping lists online so they can re-order the same items later automatically, among other features. That function is particularly useful for Sam’s business members, a segment where growth has been tentative.

“Our members are telling us it’s one of their favorite apps,” Sam’s Club Chief Executive Officer Roz Brewer told Fortune last week.

As Brewer described it to Fortune in an interview in February, the chain’s strategy for catching its rival will hinge in large part on overhauling its food offerings (grocery generates almost 60% of sales) to take on Costco’s popular Kirkland brand and win over a higher-income shopper. The median annual household income of a Sam’s Club member is $80,000, according to Sam’s Club estimates, and Brewer thinks she can get that up to $100,000. That compares to about $120,000 for Costco.

But e-commerce is also a big part of Brewer’s plan, all the more so since those well-heeled shoppers she is wooing expect such conveniences as Club Pick-Up.

Here’s how the pick-up works: it lets customers give Sam’s Club a heads up that they are on their way to the club, and at a few stores, those customers even get an assigned parking spot where they can quickly pick up an order, saving them even more time. Sam’s Club offers subscription programs online too, something aimed at winning over everyone from parents who need to constantly replenish their diaper supply to neighborhood restaurants that need a regular supply of ingredients and cleaning products.

What’s more, Sam’s Club is testing “scan-and-go” at 24 stores, with a rollout to 100 more by summer. That service will let shoppers ring themselves up on a shopping trip, with an exit greeter double checking the customers’ scanning and inspecting their shopping bags.

Sachin Padwal, a senior executive in Sam’s Club’s e-commerce business, told Fortune on a recent tour of a store in Bentonville, Ark., near Wal-Mart headquarters, that scan-and-go was about convenience and not so much about lines at the cash register. Still, much is riding on this test, since big sibling Walmart is looking to test scan-and-go too.

There is no doubt Sam’s Club’s e-commerce is growing: the web site now gets 16 million unique visits a month and sells 51,000 different kinds of items. Last year, it launched daily deals online to spur that growth. All that has given Sam’s Club a head start over Costco in the e-commerce battle. That retailer, about twice the size of Sam’s Club with $120 billion in sales last year, still doesn’t offer customers the option of placing orders online that they can fetch from a store.

At Costco, the second U.S. largest retailer after Walmart, executives have not seemed in a hurry to build up the chain’s e-commerce infrastructure. Costco Chief Financial Officer Richard Galanti told analysts on a conference call last month that the retailer was concerned about giving up incentives for shoppers to come into stores. (Costco did not respond to a request for further comment for this article.)

Costco did get $4.5 billion of revenues from e-commerce last year according to Internet Retailer, enough to make it #8 on the trade publication’s top 500 list, and is getting serious at last. (Wal-Mart does not break out digital sales by division.)

“Some out there would argue it’s about time, but we’re getting to it,” Galanti told the analysts. Indeed, in the fourth quarter. “We’re starting to do a few new things.”

That gives Sam’s Club a nice head start. But it can’t sit still: Costco’s e-commerce sales rose 19% last quarter.