Custom products e-marketplace Spreadshirt.com, which is based in Germany but sells worldwide, is making a 3 million euro (US$4.17 million) bet on mobile commerce. It’s using the funds for a product design tool makes it easier for a consumer on a smartphone or tablet to customize a garment, as well as a revamp of its web site for touchscreen devices, CEO Philip Rooke says. Some of the funds will also go to general enhancements of the company’s e-commerce platform, he adds.

Both of the retailer’s projects have already launched on the web in a basic fashion, but Spreadshirt plans to enhance them greatly over the course of the year, he says. For one thing, customers have been asking for the ability to save and view items on any device as they move between smartphones, tablets and computers, he says.

“The hard truth is that if we are not mobile—and touch-friendly—we will not meet our revenue goal,” Rooke says.  That goal is to double sales over the next three years. “If Spreadshirt does not provide a fully optimized mobile experience, then someone else will,” he says.

Spreadshirt’s mobile traffic increased 300% in 2013 over 2012, and one in five visitors now come from a smartphone or tablet, he says. By the end of the year, those devices will be involved in 25% of the retailer’s sales, he predicts. And within a few years, all of Spreadshirt’s business will likely involve at least some mobile interactions, Rooke adds, whether a customer checks for shipping confirmations via e-mail on a smartphone or receives mobile notifications about her sales. Spreadshirt allows customers to save their designs and sell them through its web store.

The mobile design tool debuted in May, Rooke says, and 3 million consumers worldwide use it each month. “We sell fun merchandizing ideas and this means that we have to adapt to the times when customers are in ‘fun’ mode,” he says. “Whether they are travelling, waiting for friends or just relaxing in front of the TV, they prefer to use a touch device—laptops are more unwieldy. This is why mobile is so important to us.”

For U.S. customers, that “fun mode” time also often includes downtime—perhaps while commuting or waiting in line, Rooke adds. That’s when much of Spreadshirt’s U.S. mobile traffic appears. Globally, mobile is also extremely popular in emerging markets like Brazil, where most consumers access the Internet primarily from smartphones, not computers, he adds. “The more nascent the commerce market is, the more important mobile is,” he says.