SEATTLE — Amazon says it will offer customers free shipping on orders delivered through Christmas regardless of whether they are members of the Prime membership program, a move that occurs two weeks after rival Target undercut the company with its own free-shipping offer.

Target last month said it would offer free two-day shipping on hundreds of thousands of items, with no minimum order size, from Nov. 1 to Dec. 22 — a move designed to keep customers looking to shop online from defecting to rivals such as Amazon or Walmart, which charge for expedited shipping.

Walmart offers free two-day shipping on sales of more than $35, while Amazon’s free delivery-window guarantees are limited to members of its $119-a-year Prime program.

On Monday, Amazon returned fire, saying it will eat the shipping costs on “hundreds of millions” of items on its online bazaar during the holiday rush.

A spokeswoman said it is the first time Amazon has dropped its minimum order size to qualify for free standard shipping — which generally takes between five and eight days — to zero.

A day after Target’s announcement, a public-relations firm hired by Amazon had sent journalists an email detailing the company’s delivery perks, a rundown that primarily listed features of the paid Prime program.

In its new announcement, the company again made the case for its flagship membership program, disclosing for the first time that more than half of U.S. Prime members live in an area served by the company’s expanding same-day Prime Now delivery service.