Exchange sales increased by 10 percent in 2010 compared to 2009, which was the result of an 11 percent increase in water bottle units sold to approximately 4.3 million units. The increase in units sold was driven by a 14 percent increase in Exchange selling locations to approximately 8,000 at Dec. 31, 2010 as well as an increase in same store units of approximately 5 percent for 2010. In addition, sales for Refill accounted for $3.3 million since the acquisition on Nov. 10, 2010 from approximately 4,600 locations. Product sales decreased $8.1 million or 35.4 percent to $14.7 million, representing 33.0 percent of total net sales for 2010. We believe the decrease in sales is primarily the result of retailers continuing to manage their inventory levels in anticipation of the new product line, which the company began shipping in the fourth quarter of 2010. Product sales in the fourth quarter of 2010 increased 38.3% compared to prior year. Product sales at retail to end consumers increased 14 percent in 2010 compared to 2009.
The company's overall gross margin increased to 23.3 percent in 2010 from 17.5 percent in 2009, primarily as a result of a higher mix of high margin water sales, which represented 63.3 percent of sales in 2010 compared to 48.2 percent in 2009. SG&A expenses increased $2.7 million or 27.2 percent to $12.6 million for 2010, which is primarily due to an increase in headcount and related costs associated with becoming a public company, costs related to the company's Refill acquisition and costs of operating duplicate back-office operations.