Updating this since it’s been a while. We’re still leaving this as a possible future improvement. This is not a very popular feature on our website. Figuring out the design for mobile apps continues to be a challenge. Sorry for the inconvenience!Eric Luce commented
I understand the design challenge this poses for a mobile app, with there being a significant amount of data entry required, but I think it's important to include this feature on mobile because otherwise I have to do the math myself when sharing certain groceries between my roommates.
Maybe you could set it up so that we enter them like normal transactions but with a way to group them so that tax and tip can be included and split accordingly.