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 and fulfillment costs associated with seller transactions are based on the gross purchase price of underlying transactions, and payment processing and related transaction and fulfillment 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 2015 , 2014 , and 2013 , compared to the comparable prior year periods, is primarily due to variable costs corresponding with increased physical and digital product and service 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 a greater selection and in-stock inventory levels and to 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 regularly evaluate our facility requirements. 

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, direct sales, 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 2015 , 2014 , and 2013 , compared to the comparable prior year periods, is primarily due to increased spending on online marketing channels, as well as payroll and related expenses. 

While costs associated with Amazon Prime memberships and other shipping offers are not included in marketing expense, we view these offers as effective worldwide marketing tools, and intend to continue offering them indefinitely. 

Technology and Content 

Technology costs consist principally of research and development activities including payroll and related expenses for employees involved in application, production, maintenance, operation, and platform development for new and existing products and services, as well as AWS and other technology infrastructure expenses. Content costs consist principally of payroll and related expenses for employees involved in category expansion, editorial content, buying, and merchandising selection. Digital media content costs related to revenue recorded gross, including Prime Video, are included in cost of sales. 

We seek to invest efficiently in several areas of technology and content so we may continue to enhance the customer experience and improve our process efficiency through rapid technology developments while operating at an ever increasing scale. Our