For questions two and three, if you don't work in customer service you can imagine you do or draw on previous experience.
You get home from shopping and discover you've been overcharged a couple of dollars/Euros/pounds/[other local currency]. Do you go back and get it refunded?
You get home from work, where you've also bought something before leaving, and realise you've been overcharged a couple of [local unit of currency] by the colleague who served you. Do you get it refunded next time you go into work?
How do you feel about shopping at your own store?
If you've worked in customer service, what kind of staff benefits did you/do you get?
If you answered "other", feel free to specify in a comment.