Beyond the numbers, Thompson says members are widening their spending－in much the same way shoppers patronize adjacent storefronts at the mall.

Cross-shopping, when members who start out buying from one or two retailers evolve to shop from more and more, grew nearly 60 percent in 2014, from just under 20 percent previously.

"It's that age-old tenant in the mall that draws customers in, and people around it benefit from the traffic," Thompson said.

Over the longer term, Thompson hopes to take greater advantage of ties to Alibaba, the Chinese e-commerce titan that paid $202 million for 39 percent of the company in 2013.

ShopRunner is one of several US e-commerce companies working with Alibaba and affiliate Alipay to help US retailers access China.

Through Alipay's nascent "ePass" service, which aims to let Chinese customers buy and pay for goods from US retailers hassle-free, ShopRunner helps Neiman Marcus and Cole Haan sell to consumers in China by marketing their brands to buyers there.

As Alibaba's biggest single US e-commerce investment, ShopRunner will play a key role in Alipay's effort, although it also has relationships with sites in which Alibaba has no equity holding.

Thompson would not give specifics but said ShopRunner's own four-month effort to sell through Alipay was going well and demonstrated the vast potential of a market in which retailers like Best Buy have made little headway despite years of effort.

There's some evidence Alibaba's program is taking off. Gilt.com has seen a doubling in sales to China since it started working with Alibaba and Alipay.

And iHerb, a little-known purveyor of vitamins and health supplements, has seen sales surge 693 percent in the first six months, Alipay Americas president Jingming Li told a conference in San Francisco on Wednesday.

"This will be, over a three-year period, equally as big a business and equally as compelling," Thompson said.

"It's still early. The opportunity is just too big to miss getting this one right."