party goers can scan a QR code that allows them to send a song request and leave a tip or just leave a tip 
Since we can sync our whole library into Lex, Lex can allow the party goer to pick a song from our library to pay us to play.
We can accept the request or reject it.
Lex gets the payment and takes a percentage to complete the transaction. Lex then sends the rest to us via our accepted payment method.
We get notified viz the Lex mobile app.
If the Lex app can read that we have access to streaming services it can tell us if the song request is in the library or in our streaming service if possible.
Doing anything with payments is way outside of what Lexicon or the mobile app does.
Song requests don’t need a mobile app so that could even be done with just Lexicon, but not really something important to add I think. Especially if you have a streaming service, you can basically fill any request so no need to know your music library.