That costumer is an absolute asshole. If he wanted Shuu to add his own touch, why not tell him upfront? It could be that easy. But no, he's got to be secretive about it and only ever reveal it once the work is done so he can simply shit on Shuu's work. And my guess is, the same would've happened if Shuu actually did give the kettle his own touch. But in that situation the costumer would've had a much better case for himself since the delivered work wouldn't match the requested work.