Waitrose pays quite well I believe, even for 16 year olds. I don't remember the exact figure but its certainly one of the highest. Guess you could count them as luxurious food supermarkets. You should be able to ask them the expected salary but just ask them as gently as possible.
Supermarket assistant - right off the bat, they will ask you about how to deal with customers, how to help them, how to deal with any customer problems, so its basically a very 'customer facing' position. Go on youtube and look for interview questions to do with customer facing roles. There should be general shop assistant interview questions and that will be more than enough.
Teamwork is another element they will question you on, I understand paper round is a pretty solo role but you could talk about working in a group at school, if not just tell them how you're great with people and good at communicating and connecting/being open and friendly.
Is there no job description for this role?