Walmart is teaming up with DoorDash[1] to support its ambitious nationwide grocery delivery program.

The world's largest retailer said Tuesday that DoorDash will deliver Walmart's online grocery orders to customers, starting with the Atlanta metro area and expanding to more markets in the coming months.

The partnership is part of a mashup of delivery options Walmart has turned to as it works to expand its grocery delivery program to more than 100 metro areas across the country by the end of this year. The push will make Walmart's delivery service available to roughly 40 percent of US households, the retailer claims.

Grocery delivery has been a key focus for Walmart over the last two years as it's tinkered with the economics of various programs. Before committing to a national expansion, Walmart had to first figure out how to cut shipping costs and make grocery delivery a more efficient and scalable part of its business.

To that end, Walmart partnered with Uber in 2016 when it kicked off a grocery delivery pilot program that also included Lyft and Deliv. The retailer experimented with other last-mile delivery options since then, including a pilot[2] that has store associates delivering online orders on their way home from work. Walmart also recently inked a delivery partnership with Postmates.

Walmart's delivery system is still a work in progress, but the company appears committed to using a combination of crowd-sourced delivery services to make the program come together.

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