I would ignore the buyer, open an unpaid item dispute after 4 days and then the only way they can avoid a strike and leave you a neg is to pay up in full. If they do that then I would wear it with pride and a well worded follow up. Of course if they don't pay you close the dispute after 4 days and then feedback is no longer on the table.

If they won't fit in a B4 box or your box won't fit in a 5kg satchel it is unfortunate but you have done exactly the right thing.