in units shipped, driven by increases in units shipped in Japan and Germany. For the three months ended June 30, 2021, QVC-International experienced shipped sales growth in constant currency across all product categories. QVC-International net revenue growth in constant currency for the six months ended June 30, 2021 was primarily due to a 3.5% increase in ASP, driven by ASP increases across all markets, and a 4.1% increase in units shipped, driven by increases in units shipped across all markets except for the U.K. For the six months ended June 30, 2021, QVC-International experienced shipped sales growth in constant currency across all product categories.
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 63.1% and 64.0% for three and six months ended June 30, 2021, respectively, compared to 63.7% and 64.4% for the three and six months ended June 30, 2020, respectively. The decrease in cost of goods sold as a percentage of revenue for the three and six months ended June 30, 2021 is primarily due to favorable estimated product returns at QxH and Germany, strategic promotional and pricing initiatives, which decreased product costs as a percentage of net revenue in Germany and Japan, a shift in product mix to higher margin category of apparel and decreased obsolescence as a result of less aged inventory at QxH. These decreases were partially offset by increased freight charges and warehouse expenses 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 increased $9 million and $18 million for the three and six months ended June 30, 2021, respectively, as compared to the same periods in the prior year. The three month increase is primarily due to a $4 million increase in customer service expenses primarily at QxH, a $3 million increase in commissions, primarily at QxH and to a lesser extent, Japan, and a $3 million increase due to unfavorable exchange rates. The six month increase is primarily due to an $8 million increase in customer service expenses primarily at QxH, a $4 million increase in commissions, primarily at QxH and to a lesser extent, Japan, and a $6 million increase due to unfavorable exchange rates.
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 were flat and decreased $2 million for the three and six months ended June 30, 2021, as compared to the same periods in the prior year, and as a percentage of net revenue, decreased from 10.8% to 10.6% and from 11.1% to 10.3% for the three and six months ended June 30, 2021, respectively, as compared to the three and six months ended June 30, 2020. For the three months ended June 30, 2021, there was a $20 million decrease in credit losses, primarily at QxH, and a $7 million decrease in personnel costs, primarily at QxH. These decreases were offset by a $20 million increase in online marketing primarily at QxH and a $7 million increase due to unfavorable exchange rates.
For the six months ended June 30, 2021, the decrease was primarily due to a $45 million decrease in credit losses, primarily at QxH, and a $4 million decrease in personnel costs, primarily at QxH. These decreases were offset by a $38 million increase in online marketing primarily at QxH and an $11 million increase due to unfavorable exchange rates. The decrease to estimated credit losses for the three and six months ended June 30, 2021 was due to a decrease in the number of installment counts offered to and taken by customers, an increase to our reserve as a result of COVID-19 for the three and six months ended June 30, 2020, favorable adjustments based on actual collections and enhanced risk screening. The decrease related to personnel costs for the three and six months ended June 30, 2021 was primarily due to a reduction of 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 $11 million and $20 million of stock-based compensation expense for the three and six months ended June 30, 2021, respectively, and $10 million and $16 million for the three and six months ended