Looks good! Loving the integration.
Out of interest - what did they go for? Even splitting or choosing what you owe?
Personally I love the idea of being able to check the items each person owes, as I’m always the one who doesn’t get a starter or dessert and end up being all socialist and subsidising my mate’s vast eating habits 
More info on this here: Deliveroo integrate with Monzo to Split the Bill
Ah - sadly a straight split (which never really works, someone always loses out!) but still, a great step forward 
Couldn’t you just ask the OS, “Is Monzo installed?” and open the Monzo app saying “split a bill of X amount”? I think technically this works on Android and iOS, as both platforms have apps which seem to do this (Gmail on iOS and Android Intents)
Asking for the username feels like quite a bit of effort which doesn’t even need to happen (Deliveroo don’t even need to know my Monzo username!)
I’ve never added up individual items to see what I owe. Have always split on based on how many people eating. The thought had never even crossed my mind - until now. I wonder if I’m a winner or loser.
That aside - great to see some big name integrations.
I’ve only done it when someone has had significantly less or more than others. Generally it balances it out I reckon.
This does seem slightly clunky, but I’m excited about such use cases. I second the idea of having the choice to split evenly (i.e. 3 ways) or alternatively select some items and assign the cumulative value (plue a proportion of delivery cost) to a specific user… the items could then even be t̶i̶c̶k̶e̶d̶ ̶o̶f̶f̶ ̶t̶h̶e̶ ̶l̶i̶s̶t̶ with the related 
username next to it.