Thank you very much for this very much needed contribution, great work! As I will heavily rely on it I tested it intensively and founded few minor issues, sure fast to fix.

In all 3 tests of the stock level you test against the threshold. But the threshold is for informing admin that only few pieces are left, so you should test against 0. See patch.

Second is about your use of the t()-function which is not ok. See the patch how to get variables into the message.

Thanks again for this important contrib,

Al