When creating a new parcel in Postmaster (1.2 beta, build 22.214.171.124), I always find that after selecting a channel, the list of categories for that channel never updates. The text remains as, "This channel has no categories." Even though there are categories for the channel that I selected.
The statuses do update, so I wonder if I'm missing something from the setup as to why these categories don't appear right away? After saving the parcel and going back into it, the categories do then appear.