I think S/S is a great single since I happen to like both songs, but I think the single lacked some of those exclusive goodies that can be included... not to mention it had 2 versions. Say if S/S had another version, maybe it'd have an additional 5k copies sold, so it'd stand at 112k right now instead.
Judging from how well S/S did in the download charts, I'd say it had some potential to sell well... but then since the 10th anniversary year of good sales, our judgement has been clouded because MW and D/G sold really well.
Before the 10th anniversary, Ayu's last physical single in 2007 was T2M right? And that didn't even hit 110k. And Glitter/fated sold moderately. I think S/S is doing normally in this time and age, but the 10th anniversary has made us think it's not selling well.
10th annniversaries are great for marketing.