Shipt, a grocery delivery start-up, is expanding its partnership with Costco and plans to double its overall market size this year.
Shipt announced Thursday it will add nine Florida markets — six markets with Costco home delivery and three with regional grocer Publix. CNBC reached out to both retailers for comment.
At the same time, a new report released Wednesday by Wells Fargo reveals most consumers still are "reluctant to shift food purchases online."
"Our biggest competitor is really the habits that consumers have developed over so many years," said Shipt founder and CEO Bill Smith, who started the internet-based service in 2015. "We have to convince customers to try grocery shopping a different way."
Smith said the company has focused its efforts on service and "delivering a really high-quality experience so we can retain customers." He said the effort seems to be paying off with Shipt "running neck and neck" with AmazonFresh in terms of share of wallet spend.
"Literally, every month it switches back and forth," said Smith. "We're growing at a faster pace than all these companies."
According to Second Measure data, Shipt had a share of wallet spend in the online grocery delivery segment of nearly 20 percent as of March 1, while AmazonFresh during the same period was 23 percent.
Yet Shipt beat AmazonFresh in share for several months during 2016. CNBC reached out to Amazon for comment for this story.
"We took a very different approach to this business," said Smith, explaining that most of the rival companies are mainly focused on the coasts while Alabama-based Shipt is going for the "mainstream, hometown America type of customer."
Added Smith, "What I tell people is our customers are a lot more likely to be buying Cheerios than they are kale."
Shipt supplied the Second Measure share information, which is based on anonymized data from billions of consumer credit card transactions. It provides data to show exactly where consumers are spending their money.
To be clear, the share data provided to CNBC tracks four companies in the grocery delivery service category: AmazonFresh, Google Express, InstaCart and Shipt. It doesn't appear to include FreshDirect, an online grocery delivery service known in the New York market.
As for InstaCart, the San Francisco-based company holds a dominant share of just more than 50 percent, according to the Second Measure data. Shipt is a little over 40 percent of the size of InstaCart, based on share but still far from its rival in terms of overall markets served.
Shipt closed out 2016 with 30 markets served, and the company plans to reach 60 markets or more by the end of 2017. With the new launches announced Thursday, the company already has about 40 markets.
InstaCart, which got a three-year head start and has raised far more money from outside investors, is currently in about 50 markets and expects to close the year "somewhere between 85 and 100 markets," according to Nilam Ganenthiran, the company's chief business officer.
"By 2018, 80 percent of the households in the United States will be able to order [groceries through] InstaCart," said Ganenthiran. "We're growing extremely fast."
InstaCart works with 160 grocers across the country, including Whole Foods Market, Kroger, Costco and many others. The online service even has some retailers such as Whole Foods as an investor.
"The mission that we're on is we're trying to enable brick-and-mortar grocers to get online to compete in an increasingly Amazon world," said Ganenthiran. He also pointed out that the company isn't just focusing on the coasts but growing in Texas, Florida and mid-Atlantic states as well as Midwest markets such as Missouri, Indiana and Illinois.
Besides same-day delivery, InstaCart also offers a so-called click-and-collect service where customers can pick up online orders curbside at many locations. Shipt plans to launch curbside service in a few months with one of its retail partners.
Meantime, brick-and-mortar supermarket giants such as Wal-Mart Stores and Kroger's ClickList service are also expanding their online order and store pick-up programs.
Wells Fargo analyst Zachary Fadem estimates Wal-Mart has about 13 percent or more of its U.S. stores with a click-and-collect service. As for Kroger, he estimates almost a quarter of its stores offer pick-up service.
"E-commerce represents the fastest growing channel within food retail," Fadem said in a research note Wednesday. However, the analyst pointed out that the online grocery category still is "underpenetrated at just 1 to 2 percent share of total grocery spending."
Wells Fargo's research concluded that "consumers appear more resistant" to online grocery services than investors may realize. It cited a recent survey by the research firm showing 82 percent U.S. consumers are still "reluctant to shift food purchases online."
Even so, the analyst said the research shows about one-fifth of people surveyed had purchased food on the internet.
"I would say that online grocery is still in its infancy stages," said Fadem. "Investors are obviously anticipating it will be larger. But the question is whether it will play out over a 10-year period or a period much shorter than that."