I've done the same - the Post Office destroyed the package and I had to refund the buyer. Luckily it was only £15 or so.FWIW
I sold an Li Ion battery to a member here.
I had the original plastic case and & OEM box. When I presented at the PO counter, I was asked and told the assistant that it was a battery in the manufacturers packaging. She accepted the parcel but then stuck the "battery warning" label on it.
The recipient confirmed that it had arrived safely & sound. NB I sent it as "Signed For"