in our inventory, and recognized as cost of sales upon sale of products to our customers. 

The increase in cost of sales in absolute dollars in 2011, 2010, and 2009 compared to the comparable prior year periods is primarily due to increased product, digital content, including Prime Instant Video, and shipping costs resulting from increased sales. 

Gross margin for 2011 remained relatively consistent with the prior year periods. We believe that income from operations is a more meaningful measure than gross profit and gross margin due to the diversity of our product categories and services. 

Fulfillment 

Fulfillment costs as a percentage of net sales may vary due to several factors, such as payment processing and related transaction costs, our level of productivity and accuracy, changes in volume, size, and weight of units received and fulfilled, timing of fulfillment capacity expansion, the extent we utilize fulfillment services provided by third parties, mix of products and services sold, and our ability to affect customer service contacts per unit by implementing improvements in our operations and enhancements to our customer self-service features. Additionally, because payment processing costs associated with seller transactions are based on the gross purchase price of underlying transactions, and payment processing and related transaction costs are higher as a percentage of sales versus our retail sales, sales by our sellers have higher fulfillment costs as a percent of net sales. 

The increase in fulfillment costs in absolute dollars in 2011, 2010, and 2009, compared to the comparable prior year periods, is primarily due to variable costs corresponding with physical and digital product and services sales volume, inventory levels, and sales mix; costs from expanding fulfillment capacity; and payment processing and related transaction costs. 

We seek to expand our fulfillment capacity to accommodate greater selection and in-stock inventory levels and meet anticipated shipment volumes from sales of our own products as well as sales by third parties for which we provide the fulfillment services. We evaluate our facility requirements as necessary. 

Marketing 

We direct customers to our websites primarily through a number of targeted online marketing channels, such as our Associates program, sponsored search, portal advertising, email marketing campaigns, and other initiatives. Our marketing expenses are largely variable, based on growth in sales and changes in rates. To the extent there is increased or decreased competition for these traffic sources, or to the extent our mix of these channels shifts, we would expect to see a corresponding change in our marketing expense. 

The increase in marketing costs in absolute dollars in 2011, 2010, and 2009, compared to the comparable prior year periods, is primarily due to increased spending on online marketing channels, such as sponsored search programs and our Associates program, payroll and related expenses, and in 2011 and 2010 television and print advertising. 

While costs associated with Amazon Prime memberships and other shipping offers are not included in marketing expense, we view