I would replace the inventory feature with an age feature, because I think for most people tracking how many weeks/months ago you bought the tea is more important than how many cups you have left.
Also, while I conceptually like the features the UI isn't especially aesthetically pleasing. I would also recommend adding the preferred units to a preferences page and eliminating that from the brew page.
Age does seem more relevant than volume left! Although you have no way of knowing how long it sat in the supply chain before you bought it (unless they time stamped it).
While this is true, decent tea will be well packaged and should stay fresh until one cracks it open. Oolongs and other quality teas that are best fresh tend to be sold with the date of harvest (like Alishan Winter 09 (sigh!)), while pu erh can, and arguably should, be 12 years old when one drinks it.
In fact, for serious tea heads, tracking the age of tea might be most useful for aging pu erh disks that aren't quite 'there' yet.
Also, while I conceptually like the features the UI isn't especially aesthetically pleasing. I would also recommend adding the preferred units to a preferences page and eliminating that from the brew page.