Consumers purchased $50.2 billion via their computers from online retailers in the first quarter of 2013, a 13% increase from the same quarter a year ago, web measurement firm comScore Inc. reports today.

In addition, shoppers bought $5.9 billion worth of retail goods via their tablets and smartphones, says comScore, which reported mobile commerce sales for the first time.

Although e-retail’s year-over-year spending increase of 13% for the quarter bodes well for e-retail spending over the rest of the year, it does slightly lag the year-over-year increases of previous quarters, says comScore chairman Gian Fulgoni. In Q4 2012, U.S. e-retail spending grew 14%; in Q3 and Q2 it grew 15%; and in Q1 it grew 17%.

“One potential explanation for this mild deceleration is the payroll tax increase, which went into effect in 2013 and which removed some disposable income from Americans’ wallets,” Fulgoni says. “That said, as long as job growth continues and consumer sentiment remains positive, the outlook for e-commerce in 2013 remains bright.”

However, the possibility of a federal sales tax law that would require e-retailers to collect state sales tax on every transaction may hinder e-commerce’s continuing growth rate in the future, he adds.

When adding together the purchases made on PCs along with those on mobile devices, online sales for the quarter total $56.01 billion. ComScore has not disclosed its m-commerce estimate for the first quarter of 2012, but Andrew Lipsman, vice president of marketing and insights, says retail purchasing on smartphones and tablets is growing at about twice the rate of buying via PCs.

Assuming a 26% growth rate for m-commerce over the past year—double the 13% growth of desktop e-commerce—that would suggest Q1 2012 mobile sales were around $4.7 billion, and total online sales would have been nearly $49 billion. Comparing that figure with the $56 billion in first quarter 2013 total online sales would yield a growth rate of just over 14%, about a percentage point higher than the 13% increase in desktop sales. That still would represent mildly slower growth, as the growth rates for recent quarters have all been understated because comScore has not been including purchases made on tablets and smartphones. Lipsman says he and Fulgoni will provide more detail on mobile commerce growth during a comScore webinar Tuesday.

Consumers in the first quarter spent 10.6% of their discretionary spending with online retailers, the highest share to date and up from 9.4% in the first quarter of 2012, comScore says.

Five product categories each registered year-over-year sales increases of 20% or more in the first quarter: digital content and subscriptions, apparel and accessories, sport and fitness, consumer electronics and consumer packaged goods, comScore says.

Moreover, U.S. consumers spent nearly half their time (48%) shopping the web from mobile devices, the firm says. That includes 34% of their time browsing on smartphones  and 14% on tablets.

 

Favorite