UPDATE (4/10): Walmart today launched its same-day grocery delivery service in Charlotte, N.C. The retailer also announced that courier service Postmates is joining its delivery network.

"With the help of Postmates, we're making grocery shopping even easier by bringing the everyday low prices of Walmart right to the front door of customers in Charlotte with more areas to be added soon," Walmart US Executive Vice President of Central Operations Mark Ibbotson said in a statement. Besides Postmates, Walmart works with Uber and Deliv to get orders shuttled to customers.

The expansion comes after Walmart last month announced plans to grow the service to serve more than 100 metro areas across the country by the end of the year.

Original Story (3/14):

Time to step it up, Amazon. Rival Walmart just announced a major expansion of its online grocery delivery service.

Currently available in just six markets, the service, which lets people order groceries online and have them delivered the same day, is slated to reach 100 metro areas across the country by the end of the year, Walmart announced Wednesday. Following the expansion, the service will cover 40 percent of US households.

"We're saving customers time by leveraging new technology, and connecting all the parts of our business into a single seamless shopping experience: great stores, easy pickup, fast delivery, and apps and websites that are simple to use," Walmart US President and CEO Greg Foran said in a statement.

Here's how it works: just head over to the Walmart Grocery site or app, enter your zip code to see if it's available in your area, then shop the selection of "fresh produce, meat, seafood, and bakery items, along with pantry staples, consumables, and seasonal general merchandise." Once you place your order, one of Walmart's personal shoppers will gather your items and send them out for delivery. Walmart is working with Uber and other third-party crowd-sourced delivery services like Deliv to get the orders shuttled to customers.

Walmart is charging a $9.95 fee for online grocery deliveries, and your order must be at least $30. You can get your first order delivered for free with the promo code FRESHCAR.

The retailer also currently offers online grocery pickups in 1,200 stores and plans to expand that service to another 1,000 locations this year. Walmart said its personal shoppers "must complete a three week training program learning how to select the freshest produce and the best cuts of meat for online grocery customers."

Meanwhile, Amazon recently extended its two-hour Whole Foods delivery service for Prime members to Atlanta and San Francisco after first launching it in Austin, Cincinnati, Dallas, and Virginia Beach last month. Prime customers in those six cities can take advantage of this new benefit for free, or speed up their delivery to 1 hour or less for $7.99, on orders of $35 or more.