I have been contacted by Black Arrow since my post on 18 May 2014. They induced me to sign an undertaking to not to defame them, and promised me to refund immediately. I signed, but then they refused to refund again.
I am bringing this to the solicitor and HK Police.
Good luck to us all.
P.S. note that the Hong Kong address Black Arrow used on their invoice may be fake. It is used by MANY MANY others on the internet, just search it on google and you will find many results.
Why would you sign something like that?
If they have not paid, and you are not telling a lie, then post a copy of the undertaking.