U.S. e-commerce sales totaled $47.51 billion during the second quarter, up 17.5% from $40.42 billion a year ago, according to seasonally adjusted estimates released today by the U.S. Commerce Department.

E-commerce accounted for approximately 4.6% of total retail sales during the second quarter, its highest level on record, the Commerce Department says. Total adjusted retail sales excluding foodservice during the second quarter reached $1.04 trillion, according to today’s report. E-commerce accounted for 4.5% of total retail sales during the first quarter of 2011, and 4.2% of total retail sales during the second quarter of 2010, the Commerce Department says.

When further excluding sales in categories not commonly bought online—automobiles, fuel and grocery—Internet Retailer calculates that e-commerce accounted for 8.4% of total retail sales during the quarter, up from 7.3% a year ago.

On an unadjusted basis, the Commerce Department says e-retail sales excluding foodservice during the second quarter totaled $44.22 billion, up 17.2% from a year ago. It says e-commerce accounted for approximately 4.2% of the quarter’s unadjusted total retail sales of $1.06 trillion. Following the same method outlined above, Internet Retailer calculates e-commerce accounted for 7.8% of unadjusted retail sales of items consumers often purchase online.

The Commerce Department numbers are higher than e-retail sales and growth estimates released earlier this month from comScore Inc. The web measurement firm reported consumers spent $37.5 billion with online retailers during the second quarter, up 13.6% from $33.0 billion during the same period in 2010. ComScore draws on online purchase data from its panel of about 1 million U.S. online shoppers and excludes automobile and auction sales. Commerce Department estimates are based on a quarterly survey of more than 11,000 U.S. merchants.

While April through June total e-commerce and retail sales were up from a year ago, some retailers are likely to carefully watch their expenses and inventory levels given the recent swings in the stock market and continued economic uncertainty.

For instance, Saks Inc. today forecast that it would raise inventory levels by a mid-single digit percentage during the second half of 2011 compared with the same time a year ago, while it expected store sales to rise at a slightly higher rate. Keeping a tight rein on inventory levels would help prevent the retailer from having to cut prices to move excess inventory should the economy remain volatile. Saks Direct, the e-retail arm of Saks Inc., is No. 38 in Internet Retailer’s Top 500 Guide.  

“We are approaching the fall season a bit more cautiously and will continue to be very strategic with our expense, capital and inventory spending,” says Stephen Sadove, CEO of Saks Inc., where online sales were up 50% during the second quarter from a year ago, and bricks-and-mortar comparable store sales were up 15.5%.

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