Small business sent me someone’s else’s order and wants me to pay to ship it back to them…….

ok so i ordered some socks online from a smaller business and they sent me someone’s else order. their name and address and everything included. i contacted customer support and they’re shipping my correct order back to me but they asked me to mail back the other order. they didn’t give me a shipping label. they expect me to pay out of pocket to box and ship back the wrong order. am i wrong if i tell them to eff off and keep the order? it was their mistake in the first place and why am i being expected to pay out of pocket to send it back to them?