Opt-In tipping instead of Opt-Out.

There's a big difference between asking how much you want to tip versus giving the option to tip if they press an optional button. Especially for services that don't usually but would sometimes like to tip (such as retail).

Leaving customers with thinking about if the person behind the counter is judging them for not tipping is not the final impression a customer should be left with and that is the only option with opt-out tipping being the only tipping option.