The "insufficient funds" error is hard to find, disappears rapidly, and could perhaps be clarified now that we have two currencies:
1. The "insufficient funds" error appears above the confirm button... aka, on top of the "empty cart" button. Furthermore, the confirm button has a tool tip that appears in the same place. As a result, failed purchases show an exclamation point, but the text of error message disappears almost instantly. To fix this, the error text should be moved to a less crowded location, and given a longer lifetime before vanishing.
2. There is no difference in the error messages for "insufficient crowns" or "insufficient energy"... adding separate notes for each would help clarify player purchases.
(It might also have saved me from a boneheaded mistake purchasing a trinket slot, when I didn't look closely enough at the currency label. I just wiped out all the energy I'd allocated for beta testing tonight... whoops!!)
It's as noticeable as it could be without a message that shouts at you saying: CHECK YOSELF