[derefr]

Here in Canada, portable POS terminals usually have a screen before "TAP/INSERT CARD" that says "Add tip?" and offers vendor-set default options, usually either 10%/12%/15% or 12%/15%/20%. The fourth option is always "custom", which allows people to enter 0 if they wish. (There are also payment flows that have an initial "Yes/No" on the "Add Tip?" question, in cases where a tip may or may not make sense, e.g. a bakery/deli that offers both high-touch dine-in and extremely-low-touch "we just take it from the fridge and give it to you" take-out service.)

Since Canada and the US have basically the same tipping culture, these POS systems seem to be already tailored for adoption in the US market (or at least, being cloned by US POS mfgrs.) Not sure why they haven't been.