She should have been emailed a label from Ebay. Once she has printed the label, she will need to drop the shoes off somewhere - hopefully, Ebay will supply instructions. I copied and pasted instructions below - I assume 'DPS' should read DPD??
Shipping sneakers with Authenticity Guarantee:
UK buyers pay a flat £8.95 eBay postage fee when purchasing sneakers that qualify for Authenticity Guarantee. We'll then send sellers a pre-paid DPS postage label so they can post the sneakers to the authentication centre, free of charge. If you have a buyer with a multi-quantity order for the same item and are unable to print enough shipping labels through your eBay account, please contact our customer support so we can assist.
"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)