Challenging Amazon, Walmart starts offering one-day shipping service.

The new free NextDay delivery offering is available on a wide range of general merchandise from Walmart.com, without a membership fee. NextDay delivery will be available first to Walmart.com customers in Phoenix and Las Vegas, and will expand to Southern California in the coming days. It will roll out gradually over the coming months, with a plan to reach approximately 75% of the U.S. population this year, which includes 40 of the top 50 major U.S. metro areas.

Walmart’s NextDay delivery includes up to 220,000 of the items most frequently purchased, ranging from diapers and laundry detergent to toys and electronics, with more assortment to be added. It is available on eligible orders of $35 or more, with the specific assortment varying by customer location.

Amazon last month announced plans to upgrade its Prime membership program in the US from two-day shipping to just one day. There's one more very big difference between these competing programs: Amazon's requires a $119 annual membership, while Walmart's is free, so long as orders are $35 or more.

NextDay delivery makes business sense, as it will cost Walmart less – not more – to deliver orders the next day. That’s because eligible items come from a single fulfillment center located closest to the customer. This means the order ships in one box, or as few as possible, and it travels a shorter distance via inexpensive ground shipping.

NextDay delivery is complementing Walmart's same-day Grocery Pickup and Delivery options, and free two-day shipping on millions of items.