Do not pay out of pocket to ship the other parcel. Ask for a prepaid label or the money up front.
Thank you, Taxel - will do!
I was intending on asking for payment up front (once I know what the total cost to send back to them is), but what are these pre-paid labels, please?
You can buy labels through USPS and UPS (probably FedEx too) online and email or link them to whoever needs to use them. I would never, ever trust a seller to reimburse me for a label after the fact, especially some random person on eBay. A real business, like an Etsy store, I might trust more if the owner was professional.
If you're not actually going to be given the package that probably won't work. I misunderstood what was happening a bit I guess! But if there's a fee to fix the seller's mess, I would require it up front. Its unreasonable for the seller to expect you to shell out for this with no guarantee they'll actually reimburse you.