- Writing about Japanese candy. - Making videos about Japanese candy. - etc.
Doing this will attract people interested in Japanese candy, and will pull in potential and real subscribers. Over the long term I think this could be good. I also think the topic of "Japanese Candy" is big enough for you to write/video about it once or twice a week.
As for the service itself - I think it lends well to word of mouth, or at least it could. People get their candy and share it with others, and those others ask where you got their candy. I was a subscriber for probably 4-5 months, and at least two of my friends signed up because of my word of mouth.
Thing is, all three of us have unsubscribed - and for all three of us, it actually came down to the quality of the candy (or perhaps, it came down to our tastes in candy). I know originally (and maybe still) your thing was about sending people Japanese candy they can't get outside of Japan too easily. That's good and all, but in the end, after a while we all realized the candy itself in terms of quality / taste was hit or miss... and with subscription, you can't have too many misses before someone unsubscribes. I'd say I personally enjoyed the candy I got half the time, so I just unsubscribed because it wasn't worth it.
I think finding candy that people will like rather than candy people find different or original is much more important. I think that's the difference between gaining more subscribers naturally through word of mouth and losing subscribers.
At least, that's my opinion on it. I do hope you start revving sales up again, though. I loved the candy when I loved the candy, and I think it's a good idea, but between myself and friends we unsubscribed because we didn't always love the candy :(