A nightmare return customer on Karrot

‌I did a shipping transaction via Karrot Market
The product is perfectly fine, but they are demanding a return just because it doesn't fit them
I said okay (I'm practically a saint at this point)
I told them to send it back with the shipping costs covered by the buyer
But then they ask me how they are supposed to do that...
They should look it up and handle it themselves, why are they asking me?