I'm gonna aim slightly higher with 20K.
Oshima Mai's debut song Mendokusai Aijou sold 11,768. The song wasn't THAT good, so I think a lot of those sales come from her fans she still has gathered during her AKB48 days.
AKB48 gained tons of popularity in the past couple of years, so the actual number of Wasamin fans could be more than we think. Secondly, Wasamin is still part of WH who are usually good for 80K~90K. Sure, a lot of that is Mayu, but Wasamin's presence in WH will have definitely helped her to gain fans.
A new addition are people who like Enka. I have no idea how popular Enka is and how much Enka songs usually sell. However, this new market in combination with Wasamin's regular AKB48 fans should be able to rack up a nice number.
Moreover, I think the song is epic.
Sadly the quality of a song isn't always reflected in actual sales.