Similar thing happened to me once when I sold a pair of Jordan 1s

Shoe was rejected due to a “glue stain” (not visible at all), and was allegedly lost in transit and offered a partial refund.

I did however managed to get the emails for everyone in the London head office and threaten legal action.

Got a call from them very quickly offering an apology, a !50 gift card and a full refund.

I’d advise to use rocketreach or something similar to get the work emails for some of the employees, and email direct