centers, negotiating better terms with our suppliers, and achieving better operating efficiencies. We believe that offering low prices to our customers is fundamental to our future success, and one way we offer lower prices is through shipping offers. 

We have aggregated our North America and International segments’ products and services into groups of similar products and services and provided the supplemental disclosure of net sales (in millions) below. We evaluate whether additional disclosure is appropriate when a product or service category begins to approach a significant level of net sales. For the periods presented, no individual product or service represented more than 10% of net sales. 

Operating Expenses 

Operating expenses without stock-based compensation are non-GAAP financial measures. See “Non-GAAP Financial Measures” and Item 8 of Part II, “Financial Statements and Supplementary Data—Note 1—Description of Business and Accounting Policies—Stock-Based Compensation.” 

In 2014, we recorded charges estimated at $170 million primarily related to Fire phone inventory valuation and supplier commitment costs, the majority of which was included in the North America segment. 

Cost of Sales 

Cost of sales primarily consists of the purchase price of consumer products, digital media content costs where we record revenue gross, including Prime Video and Prime Music, packaging supplies, sortation and delivery centers and related equipment costs, and inbound and outbound shipping costs, including where we are the transportation service provider. Shipping costs to receive products from our suppliers are included 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 2015 , 2014 , and 2013 , compared to the comparable prior year periods, is primarily due to increased product and shipping costs resulting from increased sales. The increase in 2014 was also impacted by the expansion of digital offerings and Fire phone inventory valuation and supplier commitment costs. 

Costs to operate our AWS segment are primarily classified as “Technology and content” as we leverage a shared infrastructure that supports both our internal technology requirements and external sales to AWS customers. 

Fulfillment 

Fulfillment costs primarily consist of those costs incurred in operating and staffing our North America and International fulfillment and customer service centers and payment processing costs. While AWS payment processing and related transaction costs are included in fulfillment, AWS costs are primarily classified as “Technology and content.” 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