U.S. e-commerce sales totaled $58.0 billion in the first quarter, up 15.3% from $50.3 billion in the first quarter of 2012, according to an estimate released today by the U.S. Commerce Department.

The estimate, not adjusted for seasonality, says e-commerce accounted for 5.5% of total retail spending in the quarter, up from 4.9% a year ago.  The Commerce Department includes purchases made from both PCs and mobile devices such as smartphones and tablets in its estimates. When excluding sales in categories that are rarely bought online—such as automobiles and fuel—Internet Retailer calculates that e-commerce accounted for 8.2% of total retail sales in the quarter, up from 7.3% in Q1 2012.

On a quarter-over-quarter basis, e-commerce sales declined 18.9% on a non-adjusted basis from Q4 2012. That decline is in line with previous years’ decreases as sales slow after the holiday sales season. 

On a seasonally adjusted basis, the Commerce Department says e-commerce accounted for $61.2 billion of the $1.12 trillion of total retail spending during the first quarter, or 5.5% of total retail spending. Q1 2013 e-commerce sales increased 15.2% year over year on the seasonally adjusted basis while total retail sales increased 3.7%.

Web measurement firm comScore Inc. earlier this month said shoppers spent $50.2 billion via PCs and another $5.9 billion via tablets and smartphones in the first quarter, for a total of $56.1 billion. The report marks the first time comScore reported mobile commerce sales. The $50.2 billion consumers bought online from their computers in the first quarter of 2013 is a 13% increase from the same quarter a year ago, comScore says.

ComScore makes estimates of e-commerce sales based on monitoring the online behavior of some 2 million consumes who agree to be tracked, about half of them in the United States. The Commerce Department makes its estimate based on a quarterly survey of U.S. retailers.