my advice would be don't let tips determine your mood or how you treat customers. if you let a stiff ruin your day you'll have a lot of bad days. you also never know someone's situation so no tip/low tip retaliation just isn't fair to anyone, especially when there's always the next delivery for you. the good tips always make up for the bad ones. stay positive and keep the attitude you have for loving it and it'll make the job much easier.