The Amazon-Walmart rivalry is about to enter its next logical phase.

Walmart confirmed on Wednesday that it plans to test a new unlimited shipping service for $50 a year in a move that could rival Amazon's Prime subscription service, which costs $99 a year.

The trial service is scheduled to roll out to a select group of customers by invitation only later this summer, according to Ravi Jariwala, a spokesperson for Walmart. The unlimited shipping option will apply to "more than a million" items on sale at Walmart and promises delivery in three days or less.

"We’ve heard from customers that they want shipping that's predictable and shipping that's affordable. We think there is an opportunity to put a price point that's affordable in front of them," Jariwala said.

In recent years, Walmart has tried to beat back Amazon by competing aggressively with price matching and better data and web technology.

Amazon launched Prime a decade ago and has since signed up 20 million paying subscribers. The service offers free two-day shipping along with an ever-expanding list of other perks, including exclusive access to streaming entertainment and unlimited photo storage. The goal is transparent enough: lock in customers and get them to do all of their shopping on Amazon.

If Walmart's shipping test proves successful, it could help Walmart to go after Amazon's crown jewel. Then again, Walmart hasn't had much luck in slowing Amazon's traction to date.

The Information was first to report the news of Walmart's upcoming shipping option.