Bad service and threatening a client!

I bought 2 tickets to Thailand in January 2018 with CC. After confirmation with my Bank the tickets was issued. In July/ Aug I receiced a mail from Flight Centre to say that only 1 ticket was paid for and that I must send my cc statements to them. After I queried this with the Bank they said that there was nothing done wrong on their side and the issue must be taken up with the merchandiser from the airline as they only presented/requested payment for 1 ticket. I advised the agent via e-mail and also said that the airline can contact me if they still have a problem. Another month passed and today I received a mail threatening me with hand over to legal department if I don't send cc statements!! Really? Pathetic! Do not threaten me with legal action! I did nothing wrong as a client neither did the Bank as they approved 2 transactions before the tickets was issued! I will never ever do business with Flight Centre again!