Amazon.com and Target Corp. have opened the floodgates on free shipping. Recent moves by the retailers to eliminate minimum-purchase amounts for free shipping have boosted the share of online orders that get delivered gratis, according to data from retail analytics company DynamicAction.

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 comes two weeks after rival Target undercut the company with its own free-shipping offer.