Canadian here. We tip 15% on almost everything but also pay by card most of the time. What happens is that machines have three quick press buttons (10%, 15% and 20%). You can also press the "tip $" button and enter the amount you want. Once that's done, you simply tap your card on the pay-pass machine that the barman presented to you.

That being said, I've not given money to an homeless person in ages. Whenever asked I simply answer "I only carry cards, sorry" and they immediately lose all attention towards me.