QVC's future net revenue growth will primarily depend on sales growth from e-commerce, mobile platforms, and applications via streaming video, additions of new customers from households already receiving QVC's televised programming and increased spending from existing customers. QVC's future net revenue may also be affected by (i) the willingness of cable television and direct-to-home satellite system operators to continue carrying QVC's programming service; (ii) QVC's ability to maintain favorable channel positioning, which may become more difficult due to governmental action or from distributors converting analog customers to digital; (iii) changes in television viewing habits because of video-on-demand technologies and Internet video services; (iv) QVC’s ability to source new and compelling products; and (v) general economic conditions.
QVC's cost of sales as a percentage of net revenue was 64.4% and 64.1% for the three and nine months ended September 30, 2021, respectively, compared to 63.7% and 64.2% for the three and nine months ended September 30, 2020, respectively. The increase in cost of sales as a percentage of revenue for the three months ended September 30, 2021 is primarily due to increased warehouse expenses driven by higher wages at QxH, increased freight costs and lower shipping and handling revenue at QxH. These increases were partially offset by decreased obsolescence as a result of less aged inventory at QxH. The decrease in cost of sales as a percentage of revenue for the nine months ended September 30, 2021 is primarily due to decreased obsolescence as a result of less aged inventory at QxH, and favorable estimated product returns at QxH. These decreases were partially offset by increased freight charges and warehouse expenses at QxH and lower shipping and handling revenue at QxH. Product margin for the three and nine months ended September 30, 2021 was flat due to margin favorability primarily in Japan, offset by margin pressure 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 $1 million and increased $17 million for the three and nine months ended September 30, 2021, respectively, as compared to the same periods in the prior year. The nine month increase is primarily due to a $9 million increase in customer service expenses primarily at QxH and a $5 million increase due to unfavorable exchange rates. As a percentage of net revenue, such expenses were 7.3% and 7.1% for the three and nine months ended September 30, 2021, respectively, and were 6.8% and 7.0% for the three and nine months ended September 30, 2020, respectively.
QVC's SG&A expenses (excluding stock-based compensation) include personnel, information technology, provision for doubtful accounts, production costs, and marketing and advertising expenses. Such expenses decreased $13 million and $15 million for the three and nine months ended September 30, 2021, as compared to the same periods in the prior year, and as a percentage of net revenue, increased from 10.5% to 10.8% and decreased from 10.9% to 10.5% for the three and nine months ended September 30, 2021, respectively, as compared to the three and nine months ended September 30, 2020. For the three months ended September 30, 2021, there was a $29 million decrease in personnel costs, primarily at QxH. This decrease was partially offset by an $11 million increase in online marketing, primarily at QxH, and a $3 million increase in credit losses due to favorable adjustments in the prior year based on actual collections and the release of the credit loss reserve that was recorded as a result of the additional risk due to COVID-19.
For the nine months ended September 30, 2021, the decrease was primarily due to a $43 million decrease in credit losses and a $34 million decrease in personnel costs, both primarily at QxH. These decreases were partially offset by a $48 million increase in online marketing primarily at QxH, and a $13 million increase due to unfavorable exchange rates. The decrease to estimated credit losses for the nine months ended September 30, 2021 was due to favorable adjustments based on actual collections and enhanced risk screening. The decrease related to personnel costs for the three and nine months ended September 30, 2021 was primarily due to a decrease in incentive pay for 2021 in addition to severance and a work from home allowance as a result of COVID-19, which were both recorded in the second quarter of 2020.
Stock-based compensation includes compensation related to options and restricted stock units granted to certain officers and employees. QVC recorded $13 million and $33 million of stock-based compensation expense for the three and nine months ended September 30, 2021, respectively, and $10 million and $26 million for the three and nine months ended September 30, 2020, respectively. The increase in stock compensation expense for the nine months ended September 30, 2021 is primarily related to certain officers not reaching performance targets for restricted stock units for the nine months ended September 30, 2020.
Depreciation and amortization decreased $3 million and $29 million for the three and nine months ended September 30, 2021, respectively, and included $15 million and $17 million of acquisition related amortization for the three months ended September 30, 2021 and 2020, respectively, and $46 million and $50 million of acquisition related