With less than two weeks left before Christmas, retailers and shippers are gearing up for the year-end online shopping rush and reminding customers about their cutoff dates for guaranteed package delivery by Christmas Day. And some local merchants are warning that increasing costs have made the popular perk of free shipping more difficult for them to offer.

Up to 1,000 retailers are offering free shipping with guaranteed delivery by Christmas Eve for orders placed online this Friday, also known as Free Shipping Day. The one-day shopping event, now in its ninth year, has no minimum order requirements. Participating merchants include JCPenney, Target and Babies R Us; a complete list is at FreeShippingDay.com.

“Dec. 16 gives retailers and the shipping companies enough time to make deliveries” before Christmas, said Marshal Cohen, senior retail analyst with the NPD Group, a Port Washington market research company.

Free shipping is the most-desired online promotion, cited by 71 percent of shoppers in a holiday survey by Manhattan consulting firm Deloitte.

Many major retailers, including Amazon and JCPenney, offer shipping options until Dec. 22 for delivery by Dec. 24. Amazon’s standard shipping deadline is Dec. 19, and its cutoff for two-day shipping — free for members of its Amazon Prime customer loyalty program — is Dec. 22. For JCPenney, the standard shipping cutoff is Dec. 19, express two-day shipping is Dec. 20, and for orders over $49, free next-day shipping is available until Dec. 21 at 1 p.m. and next-day shipping for $10 until Dec. 22 at 1 p.m.

UPS expects to deliver more than 700 million packages between Thanksgiving and New Year’s Eve, a 14 percent increase over last year. FedEx expects to ship about 10 percent more packages than last year. And the U.S. Postal Service projects it will deliver more than 30 million packages on Dec. 22, the busiest delivery day.

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Online sales from Nov. 1 to Dec. 5 brought in $52.16 billion, up 7.8 percent from last year, according to California software company Adobe Systems Inc. The remainder of the holiday season through Dec. 31 is projected to bring in $39.43 billion, Adobe found. The projection for just today is $2 billion, it said.

As online shopping grows in popularity, shipping costs and prices for corrugated boxes have increased. From 2013 to 2016, delivery service prices have gone up an average of 2.7 percent per year, and prices for cardboard boxes have increased at an average rate of about 1.2 percent per year, according to industry market research firm IBISWorld in Los Angeles.

Perfume.com, an online perfume seller in Hauppauge, is offering free shipping on all orders until Dec. 20. But due to the rising costs of cardboard and shipping, the company may not be able to sustain free shipping next year, president Ron Yakuel said.

“Managing increasing costs is difficult, however our volume allows us to pass extra savings on to our customers in the form of extra discounts and free shipping,” said Yakuel.

The best thing retailers can do is use smaller boxes and pouches, said Chris Cadigan, owner of Unishippers Global Logistics in Uniondale, a franchise location that sells shipping services to small and medium-sized companies and works with UPS.

“Carriers are normally charging for the space that packaging takes up in the network,” Cadigan said.

Robert Yeganeh, owner of Babylon online and brick-and-mortar shoe retailer Love My Shoes and a consultant for online retailer shoppingblitz.com, has been using heavy-duty plastic bags to save on shipping costs. Both retailers are offering free shipping for orders over $50 until Dec. 18.

“We are constantly negotiating between UPS and USPS for better rates,” Yeganeh said.