agreements. 

Sales increased 20% , 22% , and 27% in 2014 , 2013 , and 2012 , compared to the comparable prior year periods. Changes in foreign currency exchange rates impacted net sales by $(636) million , $(1.3) billion , and $(854) million for 2014 , 2013 , and 2012 . For a discussion of the effect on sales growth of foreign exchange rates, see “Effect of Foreign Exchange Rates” below. 

North America sales increased 25% , 28% , and 30% in 2014 , 2013 , and 2012 , compared to the comparable prior year periods. The sales growth in each year primarily reflects increased unit sales, including sales by marketplace sellers, and AWS, which was partially offset by AWS pricing changes. Increased unit sales were driven largely by our continued efforts to reduce prices for our customers, including from our shipping offers, by sales in faster growing categories such as electronics and other general merchandise, by increased in-stock inventory availability, and by increased selection of product offerings. 

International sales increased 12% , 14% , and 23% in 2014 , 2013 , and 2012 , compared to the comparable prior year periods. The sales growth in each year primarily reflects increased unit sales, including sales by marketplace sellers. Increased unit sales were driven largely by our continued efforts to reduce prices for our customers, including from our shipping offers, by sales in faster growing categories such as electronics and other general merchandise, by increased in-stock inventory availability, and by increased selection of product offerings. Additionally, changes in foreign currency exchange rates impacted International net sales by $(580) million , $(1.3) billion , and $(853) million in 2014 ,

2013 , and 2012 . 

Supplemental Information 

We expect our net cost of shipping to continue to increase to the extent our customers accept and use our shipping offers at an increasing rate, our product mix shifts to the electronics and other general merchandise category, we reduce shipping rates, we use more expensive shipping methods, and we offer additional services. We seek to mitigate costs of shipping over time in part through achieving higher sales volumes, optimizing placement of fulfillment 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 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.” 

We