NEW YORK (CBSNewYork/AP) — More than 30 million packages are expected to be delivered Monday on the busiest day of the year for the United States Postal Service.

Postmaster General and CEO Megan Brennan said they’re seeing a 15 percent growth in package volume over last year.

“This shows customers are increasingly counting on the Postal Service to deliver the holidays,” Brennan said.

Outside the post office in Ramsey, New Jersey, Mike Riley was trying to stuff some holiday cards into the bin, but the box was overflowing.

“I’m going to take these inside, there’s a box inside, hopefully they’ll pick them up this morning,” he told 1010 WINS’ Glenn Schuck.

“It’s crazy,” another woman said. “The line is all the way out, past the boxes.”

In order for packages to be delivered by Christmas, the Postal Service says Priority Mail must be shipped on Monday. Customers have until Wednesday to send packages via Priority Mail Express.

Thanks to online sales, FedEx and UPS have a combined 947 million packages to deliver between Black Friday and Christmas Eve — up 8 percent from last holiday season’s forecasts.

Online sales now account for 10 percent of all shopping and 15 percent during the holidays, according to research firm Forrester.

UPS plans to deliver about 36 million packages on Tuesday, its busiest day of the year, up from 35 million last year.

But the past two years have been rough for express shippers.

In 2013, they underestimated American’s growing fervor for online shopping. Throw in bad weather, and deliveries backed up. Some gifts didn’t arrive in time for Christmas.

UPS and Fedex spent heavily last year to ensure better performance, but still had some major hiccups. Staples, Toys ‘R Us, Best Buy, Crate & Barrel, J.C. Penney and Kohl’s were among the retailers who missed delivery to at least one part of the country, according to industry tracking firm StellaService.

To prevent similar mishaps, UPS and FedEx have been working with major retailers to hone their forecasts and have scheduled their extra holiday workers to better meet the shipping spikes right after Thanksgiving and the weekend before Christmas.

Some third-party tracking services have signaled a few issues with 2015 deliveries but UPS spokesman Mike Mangeot said last week that more than 96 percent of packages are being delivered on time in December and that UPS expects packages to arrive by Christmas.

While many are sending out their last-minute cards and packages, others are still shopping.

The National Retail Federation said the average holiday shopper has done about 50 percent of their shopping and 90 percent of holiday shoppers still have items to buy, including food gifts and decorations.

While physical stores still account for the majority of spending, the uneven growth between buying at locations and on websites signals the continuation of a big shift in how U.S. consumers are shopping.

That’s why many retailers are relying on deep discounts and special hours to help get last-minute shoppers through the door, including Kohl’s which is staying open 24 hours a day leading up to Christmas.

“Christmastime I do some shopping online, but most of the time I do box stores,” one Kohl’s shopper told Schuck.

Walmart will be open until 8 p.m. on Christmas Eve, the company said. Customers can order as late as 6 p.m. for in-store pick up on Dec. 23 but online delivery orders need to be placed by Dec. 22 for rush deliveries.

Target will stay open until either 10 p.m. or 11 p.m. on Christmas Eve.

Toys ‘R’ Us will remain open around the clock starting at 6 a.m. on Wednesday until 9 p.m. on Christmas Eve, the company said.

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