This has to be one of the most unusual third-party items ever released for a Nintendo product. Makes sense because Nintendo Labo is hardly typical.
As Bakulog points out, this is Cardboard Coating Spray, which coats cardboard from hand sweat, oil and grime.
Priced at ¥1706 ($20.56), there is apparently enough spray for 10 pieces worth of Labo cardboard.
No, really. This exists.