You didn't tip for X in 2003 or 2013 -- should you tip for X in 2023?

Optimus Prime said

I've seen a couple machines where there was no option for "no tip", and a few more where 20% was the lowest option

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I don't know if it's never there, but sometimes "custom" is in very small print in the corner of the screen and you really have to make an effort to go that route.