Had to return a two mode over a year ago. No problems at all. They despatched the replacement as soon as I emailed a photo of the postage receipt for the one I returned. They even credited me $10 for the hassle.
Cant fault them.
Before you do send it back, how about spending a minute to take the tail switch apart & re-build. Seems most problems are because of this, perhaps its not sealed or whatever. Mine played up after a ride, same as yours. Thinking it might have been condensation in the switch. Either way its a doddle to take apart with a sharp pair of scissors & its not played up since. Use the scissor points to turn the two holes in the collar thing. There's two to unscrew, it is simple to do.
I'm sure some chap with a wallet £400 lighter will be along to cast scorn & ridicule soon enough. 🙂