U.S. e-commerce sales totaled $51.18 billion in the second quarter, up 15.1% from $44.47 billion in the second quarter of 2011, according to an estimate released today by the U.S. Department of Commerce.

The estimate, not adjusted for seasonality, says e-commerce accounted for 4.7% of total retail spending in the quarter, up from 4.2% a year ago. When excluding sales in categories rarely bought online—automobiles, fuel and foodservice sales—Internet Retailer calculates that e-commerce accounted for 6.7% of total retail sales during the quarter, down slightly from 6.8% in the second quarter of 2011.

On a quarter-over-quarter basis, e-commerce sales increased 1.8% from Q1 2012, according to the Commerce Department.

On a seasonally adjusted basis, the Commerce Department says e-commerce accounted for $54.84 billion of the $1.08 trillion of total retail spending during the second quarter; that’s equal to 5.1% of total retail spending. It is the first time that e-commerce as a percentage of seasonally adjusted total retail spending has topped 5%.

Total retail sales, meanwhile, declined 0.4% on a quarter-over-quarter basis, while e-commerce grew 3.3%. Year over year, total retail sales increased 4.3%, whereas e-commerce sales grew 15.3%.

Web measurement firm comScore Inc. earlier this month said online retail sales totaled nearly $43.15 billion during Q2, up about 15.0% from the $37.50 billion it reported for the same period in 2011. Commerce Department estimates are based on a quarterly survey of more than 11,000 U.S. merchants. ComScore makes estimates of e-commerce sales based on monitoring the online behavior of some 2 million consumers who agree to be tracked, about half of them in the United States.