Walmart is adding more delivery companies to its roster to get groceries to shoppers' homes.

The retailer said in a blog post Thursday it's now working with Point Pickup, Skipcart, AxleHire and Roadie in cities across four states, to start, with a larger expansion to take place "in the coming weeks." Walmart currently is working with DoorDash, Postmates and Deliv. And the company says it's still on track to have its online grocery delivery service available in more than 800 stores by the end of this fiscal year, then doubling that in 2019.

Walmart continues to be a force to be reckoned with in the grocery industry, especially online.

Not only is the company experimenting with driverless cars to deliver bread and produce to shoppers' doorsteps, but U.S. e-commerce chief Marc Lore recently said Walmart will find a way to one day put groceries right into customers' refrigerators. It's clearly not afraid to push the envelope. But it also has to compete with Amazon and its Whole Foods stores.

Online food and grocery sales in the U.S. are forecast by IGD to reach $59.5 billion in 2023, up from $23.9 billion last year. That would push e-commerce sales to account for 3.5 percent of the total market, compared with about 1.6 percent today.