Probably worth saying that they should never have said same evening delivery as I bet that the bag had to clear customs first.

Raising a Customer Relations case is probably your best bet. Take screen shots of your Twitter conversation and attach it to the CR case when you submit it. And tell them that the tone of the responses was inappropriate.

They will probably give you an apology and little else to be honest. They won't however tell you what went wrong, usually they cover themselves by saying that they never tell a customer the outcome of their investigations. Which is unhelpful and frustrating to say the least when all you want is for them to learn from their mistakes.