adversely impacted, including a sharp decline in the value of the U.K. Pound Sterling as compared to the U.S. Dollar. Volatility in exchange rates is expected to continue in the short term as the U.K. negotiates its exit from the E.U. In the longer term, any impact from Brexit on us will depend, in part, on the outcome of tariff, trade, regulatory and other negotiations. Although it is unknown what the result of those negotiations will be, or whether the U.K. will leave the E.U. with any agreement as to the terms of its withdrawal, it is possible that new terms may adversely affect QVC’s operations and financial results in a number of ways, not all of which are currently readily apparent. The U.K. is scheduled to withdraw from the E.U. no later than January 31, 2020. Notably, in October 2019, the E.U. and the U.K. announced an agreement in principle on the withdrawal of the U.K. from the E.U. This agreement still remains subject to the successful ratification of the parties’ respective legislative bodies. As a result, the final terms of the U.K.’s exit from the E.U. are, and will remain for the immediate future, unclear. The U.K. may leave the E.U. without any agreement as to the terms of its withdrawal or the future economic relationship between the U.K. and the E.U. It is also possible that the U.K. will withdraw its notification to leave the E.U. or that there will be a second referendum on Brexit.
QVC's cost of sales as a percentage of net revenue was 64.5% and 63.9% for the three and nine months ended September 30, 2019, respectively, compared to 63.7% and 63.2% for the three and nine months ended September 30, 2018, respectively. For the three and nine months ended September 30, 2019, cost of sales as a percentage of revenue increased primarily due to an increase in product fulfillment costs at QxH.
QVC's operating expenses are principally comprised of commissions, order processing and customer service expenses, credit card processing fees and telecommunications expenses. Operating expenses decreased $27 million or 13% and $93 million or 15% for the three and nine months ended September 30, 2019, respectively, compared to the same periods in the prior year. For the three months ended September 30, 2019, operating expenses decreased primarily due to a $20 million decrease in commissions at QxH and a $3 million decrease in personnel costs at QxH. For the nine months ended September 30, 2019, operating expenses decreased primarily due to a $74 million decrease in commissions at QxH, a $10 million decrease in personnel costs at QxH and a $4 million decrease due to favorable exchange rates. The decrease in commissions for both comparable periods is primarily due to new longer term television distribution rights agreements entered into at HSN, which led to increased capitalization of television distribution rights agreements and favorable terms on commissions.
QVC's SG&A expenses (excluding stock-based compensation and transaction related costs) include personnel, information technology, provision for doubtful accounts, production costs, and marketing and advertising expenses. Such expenses decreased $4 million and $1 million for the three and nine months ended September 30, 2019, respectively, as compared to the same periods in the prior year, and as a percentage of net revenue, increased from 10.1% to 10.2% and from 10.0% to 10.3% for the three and nine months ended September 30, 2019, respectively, as compared to the three and nine months ended September 30, 2018, respectively. For the three months ended September 30, 2019, the decrease was primarily due to an $11 million decrease in personnel costs primarily at QxH, France and the U.K., partially offset by increases in Japan and Germany, and a $2 million decrease due to favorable exchange rates. The decreases were partially offset by a $3 million increase in outside services primarily in QxH, offset by decreases in Germany and the U.K., a $3 million increase in online marketing expenses in QxH, a $2 million increase in software expenses in QxH and a $1 million increase in bad debt expense due to increased Easy Pay usage and the number of installments taken at QxH offset by a decrease in Japan. For the nine months ended September 30, 2019, the decrease was primarily due to a $24 million decrease in personnel costs primarily in QxH, France and the U.K. offset by increases in Japan and Germany, and an $11 million decrease due to favorable exchange rates. The decreases were partially offset by a $12 million increase in outside services, primarily at QxH and Japan, partially offset by a decrease in Germany, an $11 million increase in bad debt expense and an $8 million increase in online marketing expenses primarily in QxH. The increase in bad debt expense for the nine months ended September 30, 2019 is primarily due to increased Easy Pay usage and the number of installments taken at QxH, and to a lesser extent, Germany.
Stock-based compensation includes compensation related to options and restricted stock units granted to certain officers and employees. QVC recorded $10 million and $30 million of stock-based compensation expense for the three and nine months ended September 30, 2019, respectively, and recorded $10 million and $34 million of stock-based compensation expense for the three and nine months ended September 30, 2018, respectively. The decrease in stock compensation expense for the nine months ended September 30, 2019 is primarily due to forfeitures of non-vested options from terminated individuals.