'Tis the season to be spending: When it comes to online shopping, Millennials are filling up the carts.

New research from Slice Intelligence found that in the weeks between Nov. 1 and Dec. 10, Millennials accounted for more than one-third of all online shoppers. The e-commerce research firm measured all online purchases made by consumers who use their apps (roughly 4.2 million people) by extracting e-receipt data on transactions. These apps, which include Slice and Unroll.me, help users do things like track their packages, get cash back through refunds and price dips, and unsubscribe to unwanted emails.

The study, published on Thursday, also found that online spending is up 19 percent since 2015 overall. In the past week, Amazon has increased its share of the market by five percent, and now accounts for nearly 43 percent of all shopping.

Thanks to an irregularity in the calendar this year, sales are expected to further increase--especially in the days leading up to Dec. 24th (the first day of Hanukah) and Dec. 25th (Christmas Day). Because the last brick-and-mortar shopping weekend of the season falls an entire week before these holidays, consumers are more likely to purchase last-minute gifts online.

A faster, more creative way to shop

Millennials don't necessarily prefer shopping online, but many find that it's simply less time consuming than trekking to a physical store. For Abigail Baron, Inc.'s 26-year-old assistant editor, the incentive is also monetary.

"I end up shopping online because it's more convenient, and saving money -- even just a couple of dollars -- is very important to me," she says. "I like that I'm able to price shop around various websites."

What's more, e-commerce gives customers the freedom to be more creative in their gifts, Baron explains. "I can't walk into a store and say, 'unique gifts for my 21-year-old brother,' but I can type that into Google and find dozens of options."