Amazon will offer limited free same-day delivery under its Prime shipping service as retailers try to outdo each other on delivery deals.

Amazon offers same-day delivery to Prime members for $5.99 per order and non-members for $8.99, plus 99 cents per unit. The online retailer will allow Prime members free same-day shipping on orders over $35, Greg Greeley, head of Prime, told Reuters.

“We know same-day delivery volumes will grow dramatically now that we are making it free,” he said.

Amazon’s announcement on Thursday comes just a few days after rival Walmart said it plans to test a new unlimited online shipping service this summer for $50 per year, undercutting Amazon’s annual $99 Prime shipping service.

Google also recently launched its Express shopping option that allows retailers that do not have same-day delivery capabilities, to use its network in certain markets. Google Express costs $10 a month, or $95 a year.

Prime has become the cornerstone of Amazon’s growth — and a testing ground for new services ranging from television programs and movies to delivery-by-drone.

Earlier this year Amazon said US Prime membership increased 50 percent in 2014. In December, it said customers ordered more than 10 times as many items via same-day delivery this holiday season, compared to a year earlier.

The same-day delivery service is already available in cities such as New York, Philadelphia, the San Francisco Bay area, Seattle, Atlanta, Boston, and Baltimore, among others.