There is no need to contact the seller because the seller did not ship the sneakers to you and doesn't even have your address. The seller can't do anything about that package. The authentication center shipped it to you.
When a buyer provides a bad address on eBay and package is returned to sender as undeliverable, the buyer forfeits MBG coverage. Most sellers will work with a buyer to refund less shipping/fees or to arrange re-shipping even though they are not obligated, but that's not an option here since you're dealing with the authenticity program as a middleman.
I've had a lot of RTS packages as a seller because buyers did not know they had to alert USPS in some way either by putting their name on the inside of the mailbox or filing for mail forwarding. You're not alone in not realizing you need to take steps to ensure mail delivery before using a brand new address.
I looked up on the sneakers AG page (link) and the only info I could find in the terms was:
Lost, Damaged, or Items Not Delivered.
Items that are lost or damaged during the Authenticity Guarantee process are protected by eBay's Money Back Guarantee, subject to the eMBG guidelines.
As mentioned above, you don't have MBG coverage on an item not received claim when the package is undeliverable to the address you provided.
Contact customer service and see if they have any more information to provide. I don't know how eBay's set up to handle this kind of situation with the authenticity program. Hopefully someone else will come along that has first hand experience. All I can offer is info from the policy page and advice to contact CS for more info.
Good luck!
