For this question.
You are right that eBay won't allow a do over on the closed Claim, but you are still covered by Paypal (if you use it which I highly recommend) and by the credit card you really paid with.
PP does require the buyer to pay for return shipping, however.
Discuss your options with the seller and ask for the Return Shipping Label. BTW they can buy a Canada Post label through Shippo if they are in the USA.
They can't agree if you don't ask.
For future reference.
Whenever possible, first contact the seller and ask if this can be corrected between you two.
But.
Don't get in a conversation.
If they want the item back, they send the label.
And you want MONEY,not a replacement.
Then use the money to buy the correct item.
Be polite.
Be clear about what you want and understanding about what they want.
From your point of view shipping is part of the cost.
From the seller's point of view shipping is a cost not a profit.
But again, if they don't immediately send the label, open the Claim.
But for the moment, you have up to 180 days from payment to do a credit card chargeback.