eBay Customer Service asked you to contact US ?? That is one of the best sorry worst answers CS has meted out.
We are mere members like yourself, we do not work for eBay but are volunteers, eBay members who are here to help others with selling and buying issues, but we have no access to anyone's account, just like you could not access mt account.
I'm afraid if the discount code, for whatever reason, was not added before final payment, then it cannot be added now.
@albl-8541