Who's familiar with this scenario? Karen comes up to the counter, all in a froth because she's pissed off about something and she demands you do something against company policy, which obviously you can't do. You explain this, she flips out and demands to see the manager. Manager rolls up, makes 'an exception' for Karen, making themselves look like a hero and you look like an asshole.
Well, I worked retail for 15 years and here's my advice for dealing with this situation. Remember that bit where you literally cannot do the thing Karen is demanding you do? Either you just plain can't do it without a manager's code or whatever, or you'd get in trouble for doing it? That is precisely the extent to which you should care about this situation. What the hell do you care if Karen wants to return a half-eaten bagel or a toy that her brat broke before they made it out of the parking lot? Not like it comes out of your paycheck! (At least, it had better not!) It's not your responsibility!
So make sure you don't take responsibility for the situation. Don't just tell them that you can't do the thing. Don't explain the company policy (not like they'd be listening anyway.) Don't make it seem like you're the one making the decisions here, because you're not! Tell them this:
“I can't do this without manager approval.“
It's 100% true, isn't it? You can't. It's not even passing the buck, because the buck is not yours to pass. If someone is asking for an exception to company policy, whether they know it or not, even if they're being really nice about it, that isn't your problem anymore, it's up to your manager to decide. You are not empowered to make those decisions, so don't give the impression that you are.
This phrase makes it clear that the situation is the manager's responsibility, not yours, and it introduces the manager into the conversation before the customer has the chance. It cuts a tedious and potentially unpleasant situation short and directs attention where it needs to go. Remember, you are paid to follow company policy, not enforce it!