Powera, this was my first thought as well, but even looking at the question this way is not realistic because the variables don't match. To answer this question

Powera, this was my first thought as well, but even looking at the question this way is not realistic because the variables don't match. To answer this question, we would need to test (a) credit card machine payment without tip suggestions vs. (b) credit card machine payments with tip suggestions. By comparing machine vs. cash, the fact that cash tips would never be declared accurately skews the results to where the test fails.