U.S. e-commerce sales reached new heights during the 2017 holiday season, as mobile transactions through smartphones and tablets drove a record haul for online vendors.

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Shoppers spent $108 billion in online transactions from November to December, up nearly 15% compared to the same period one year ago, according to Adobe Analytics, which tracks the e-commerce marketplace. One-third of digital holiday spending occurred through mobile platforms, which also accounted for more than half of the traffic to retail sites.

Sales on Cyber Monday, the first Monday after Thanksgiving, accounted for $6.6 billion of the holiday season tally. That total marked the largest single-day online shopping haul in U.S. history.

Low unemployment nationwide and U.S. stock market’s strong performance in recent months likely drove holiday spending, according to Adobe.

Online sales reached record levels even as brick-and-mortar retailers struggle to maintain foot traffic at their stores. Retail bankruptcies reached a six-year high in 2017, and the sector is expected to be similarly challenged this year.

Despite the retail industry headwinds, combined in-store and online sales rose 5.5% during the holidays, according to the National Retail Federation.