The headphone jack adapter is the second-best selling Apple product at Best Buy, according to Thinknum.
Joshua Fruhlinger, publisher of Thinknum, says:
"The above chart shows the Lightning-to-3.5mm Headphone Adapter's sales rank at Best Buy for the past couple years. A lower sales rank, of course, is better. As you can see, the adapter has been a top-25 seller for the better part of the past year. Apple is likely not making a ton off of the little dongles, but the fact remains that it's the company's second-best-selling product at America's number-one electronics retailer, ahead of its EarPods and Apple Watch."
Of course, this is probably a result of many people losing their headphone jack adapter, after it being released with the iPhone 7.