Once you issue a refund, any return label you have provided becomes void.
You can't send a new label through Ebay.
Depending what is being returned, you could send stamps to cover the cost of postage or you could send a stamped addressed padded envelope or postage bag to the buyer.
You can also offer to send them the cost of return postage.
In future though, never refund until you have the item back.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)