By Matt McFarland, CNN Business

(CNN) -- Walmart is investing in GM's self-driving vehicle company, Cruise, as it works to build out its delivery network.

The size of the investment was not disclosed, but it is less than $750 million, as Cruise said its more than $2 billion funding round expanded to $2.75 billion when Walmart and institutional investors joined. Cruise said it is valued at more than $30 billion.

The companies already have a partnership in Scottsdale, Arizona that will test deliveries to customers with Cruise's electric, self-driving vehicles "soon," according to Walmart. Walmart has targeted zero emissions by 2040, and was drawn to Cruise's electric fleet, the company said, which is primarily made up of all-electric Chevy Bolts.

Delivery businesses have grown dramatically since the pandemic as consumers have increasingly spent time at home, and last April, Walmart launched a service to bring orders to customers' doorsteps within two hours. Even drone delivery has been tested by Walmart and others, but technology and regulation challenges have slowed its development in the US.

Walmart competitor Amazon bought a self-driving car company, Zoox, last year, and has invested in another self-driving company, Aurora. Amazon has also tested small robots that deliver packages on sidewalks. Photos have surfaced of an Amazon trailer being pulled by the startup Embark.

But autonomous vehicles have proven much harder to develop than companies originally forecasted. Many have pushed back deadlines. Some have shifted their focus to self-driving trucks, as driving on highways is generally considered much easier than on local roads.

Walmart said that its work with Cruise is not related to its long-haul trucking fleet.

Walmart and its competitors are exploring how automating transportation can make deliveries faster and cheaper for the company, as it employs thousands of drivers that travel 700 million miles a year. Walmart already has two pilot tests running with another autonomous vehicle company Gatik in Northwest Louisiana and Arkansas. Gatik is focused on the "middle mile," trips between distribution centers and stores.