ooofI don't have any advice other than what everyone else has said, but I hope it works out for you! I'm terrified of putting myself in this situation eventually...to the point that I will package up items one at a time (print the label, package up the jeans, put the label on that envelope, stick it in my bag, then go print the 2nd label) whenever possible.
I'm a bit paranoid about this myself as I was often trucking 10-20 packages to the PO at once. I see how it's such an easy mistake! To try and avoid this I write the contents and last name on a package as soon as I've got it together, then cross-check with labels after they're printed before I put the label on top of my note to myself
This was why it took me almost 2 weeks to send back an incorrect item I received - I was crazy busy with work and hadn't been online to respond to eBay messages or provide my seller an update. I mention this to give OP some hope that her buyer will do the right thing and might just be temporarily tied up!