Safest way to send a slipcase in the mail

[Phenomenal Flea]

Hi, guys! I don't post here often, but when I was thinking of where I'd ask for advice on this matter, I thought of here first.

I bought a collector's edition from an online store, but the slipcase arrived damaged in the mail. I called up and asked for a replacement, and they'll be sending me down a courier bag with some cardboard, and they've asked me to flatten the slipcase and stick it in between. I'm imagining they'll be sending the replacement with the same cardboard, and I'm kind of worried about the cardboard rubbing against the slipcase and making the replacement look worn.

I was trying to think of ways to avoid this. Maybe put a piece of paper or some thin plastic in between? Has anybody had to do this before? Can you give any suggestions on how to avoid damaging the replacement?

Thanks, everyone.