Somewhat related: this idea of adding a "digital waiting line" on top of a normal one is horrible, the author mentions this in ATT.

Disneyland has done this with food, and it's really really irritating. Here is an example of what it is like to get Blue Milk at Star Wars Land in Disneyland:

1) order it in the app and pay for it.

2) it tells you when to go there

3) wait for your phone to turn green, with a group of other people staring at their phone. A person is there to make sure people without green screens don't get in line.

4) you are now allowed to get into a line and wait.

5) you get to the front, and you repeat the order. Someone makes it for you.

I get the desire to replace cashier's with an app. But just let people place an order, give them a number, and let them know where they are in the line.