I 'direct' the transactions at the service desk to make things smooth. Lots of prompting. Hi, how are you? Doing a return? Do you have the receipt/Do you remember how you paid? Was there anything wrong with it? Alright, you'll be getting $X. Do you want it back on the card? Do you have the card on you? You can put it in the card reader when you're ready. Be super communicative and make things clear (but only re: what the guest needs to do, don't try to explain the why of things, people don't care, they just want you to fix it). Keep up with the tasks you need to do while still acknowledging the guests in line. (Gimme one moment, I just need to throw this in the back, one second, just gotta finish writing this down, etc.) I've even had guests get upset when I step away to grab their order because... they thought I was just leaving? So now I say "let me go grab that for you" as I'm stepping away.
Uh, what else. Listen to the guest. Let them vent. Empathize. Be proactive and offer solutions. Someone's item arrived broken? I'm so sorry, that's really frustrating, do you want me to have someone grab you another one/want me to help you order another one with free shipping?
Never accuse. It's always a mistake. Oh, are you sure this is the TV you meant to return? The serial number doesn't match the box.