Iirc Buycraft's functionality outside of shop stuff is super lacking, so there'd be no way for it to be done through that avenue Though I imagine it wouldn't be too had to code something for it
Didn't we use to have a message that told us every time we logged in how long we have left of premium?
This is something that's being worked on (time permitting) I hope to have something to manage this available in the coming weeks. It was unfortunately somewhat of a downside to the Tebex/Buycraft platform. There is no means of accessing a list of ongoing / active purchases (such as premium duration) from our end. Meaning that wedw need to handle all of this logic in a plugin, as well as migrating the existing premium purchases to this new source of data and durations. That isn't to say this won't happen, just to give a little glimpse of why this isn't already the case. In the interim, I'll be having the packages reconfigured to alert players once their premium has expired. Unfortunately this can not be done ahead of time, only after the expiry.