Now that the 1st week of sales has ended, they totaled 37,008 copies sold.
All of their 1st week single sales:
Romance no Tochuu: 37,213
Ijiwaru: 39,379
Hadaka: 38,293
Black Butterfly: 37,008
It's good that they're holding fairly steady but I wonder if switching up the releases next time might help. For example, a member solo for each edition or something to boost sales. Or at the very least, a bonus item that's more interesting than a drink coaster