Wow, what a great app ! Congratulation !
This is what I love with NXT. It is "so easy" to deploy that you never know where it will emerge + talentous developers showing you things you didn't know were possible.
You should add the two screenshots above to the github README.md, they proove that it is easy to use, very well conceived, and so, worth to install/operate.
That is the kind of work the marketing fund should promote. The target is clear: slack team participants. Nxt is the tool, not the purpose of the add, the purpose is to enable tipping in slack without the need for users to register/subscribe to anything! - this is really USEFULL and according to me : NEVER SEEN BEFORE !