if not associated with individual vehicles. Taxes collected from customers and remitted to governmental authorities are recorded on a net basis (excluded from revenue).
Dealership Finance and Insurance Sales
Subsequent to the sale of a vehicle to a customer, we sell installment sale contracts to various financial institutions on a non‑recourse basis (with specified exceptions) to mitigate the risk of default. We receive a commission from the lender equal to either the difference between the interest rate charged to the customer and the interest rate set by the financing institution or a flat fee. We also receive commissions for facilitating the sale of various products to customers, including guaranteed vehicle protection insurance, vehicle theft protection and extended service contracts. These commissions are recorded as revenue at the time the customer enters into the contract. In the case of finance contracts, a customer may prepay or fail to pay their contract, thereby terminating the contract. Customers may also terminate extended service contracts and other insurance products, which are fully paid at purchase, and become eligible for refunds of unused premiums. In these circumstances, a portion of the commissions we received may be charged back based on the terms of the contracts. The revenue we record relating to these transactions is net of an estimate of the amount of chargebacks we will be required to pay. Our estimate is based upon our historical experience with similar contracts, including the impact of refinance and default rates on retail finance contracts and cancellation rates on extended service contracts and other insurance products. Aggregate reserves relating to chargeback activity were $23.5 million and $23.8 million as of December 31, 2016 and 2015, respectively.
Commercial Vehicle Distribution
Revenue from the distribution of vehicles, engines, power systems and parts is recognized at the time of delivery of goods to the retailer or the ultimate customer.
Defined Contribution Plans
We sponsor a number of defined contribution plans covering a significant majority of our employees. Our contributions to such plans are discretionary and are based on the level of compensation and contributions by plan participants. We incurred expense of $14.5 million, $16.0 million, and $17.7 million relating to such plans during the years ended December 31, 2016, 2015, and 2014, respectively.
Advertising costs are expensed as incurred or when such advertising takes place. We incurred net advertising costs of $102.5 million, $101.0 million, and $93.7 million during the years ended December 31, 2016, 2015, and 2014, respectively. Qualified advertising expenditures reimbursed by manufacturers, which are treated as a reduction of advertising expense, were $16.6 million, $17.2 million, and $14.3 million during the years ended December 31, 2016, 2015, and 2014, respectively.
We retain risk relating to certain of our general liability insurance, workers’ compensation insurance, vehicle physical damage insurance, property insurance, employment practices liability insurance, directors and officers insurance and employee medical benefits in the U.S. As a result, we are likely to be responsible for a significant portion of the claims and losses incurred under these programs. The amount of risk we retain varies by program, and, for certain exposures, we have pre‑determined maximum loss limits for certain individual claims and/or insurance periods. Losses, if any, above the pre‑determined loss limits are paid by third‑party insurance carriers. Certain insurers have limited available property coverage in response to the natural catastrophes experienced in recent years. Our estimate of future losses is