I don't want to be a dick or anything, but I think it's only natural that you only sell one once in a while.
I mean : there are only 4000 or so empegs out there. Most of the people have the lens they need, be it because they don't care for another colour than the blue that came with it or because they bought the lens 3-packs sold by empeg themselves.

It's not like these things wear out fast too. I've been using mine for 8 months now and it doesn't even have a scratch on it. So I don't think it needs much replacing.

This however DOESN'T mean that I don't commend what you're doing. Kudos to you for putitng the effort and time into it.

I've always wanted to buy one of your Neon Red lensed when I've found someone which I trust enough with a soldering iron to do the button hack for me.
I might buy it in advance anyway (knowing now that you'll most probably stop production) but I'm not sure. I think $30 (excl. shipping) is a lot of money for a lens that I don't have any direct use for.

In short : please don't stop providing this service to the empeg users. And if you can't justify the expense of having a large stock of lenses anymore, maybe you can work on a pre-order basis from now on.