It would be interesting to know what value added services they can provide to increase their margin.
To actually bring forth a useful subscription product that's delightful, and always delivered on time requires thoughtful design and implementation. Every woman is different, they require different numbers of tampons each cycle, of different absorbencies, and in their preferred brand. They don't want a backlog of products in their own bathroom that takes up room for months at a time, only to run out unexpectedly when they most need it.
This may seem like a laughable or even trivial issue to tackle, but half of the population has periods and everyone complains about them, why not do something to improve that situation?
The dollar shave club may be sustainable because they have managed to make no-name blades appealing through a viral video.