Correctamundo! One of the co-pilots on the VC10K force in its early days received a snottygram from Amex saying something of that order. I advised him to insist that OC Accounts sort it out with Amex -and that they must send him a letter confirming that he was absolved from any personal blame, the matter was entirly between his employer (RAF) and Amex. He eventually received such a letter.
RAF bill settling is notoriously slow and has led to all sorts of down-route problems in the past - such as catering companies refusing to supply the RAF due to previous instances of late payment.