Interesting problem! I agree your best bet is to ring, not write, and hopefully explain how unfair/absurd the outcome would be to deny you compensation.
A strict literal reading of the terms does not exclude your claim, in my opinion, because it says 'when' and not 'if'; 'when' seem to indicate only the time customs has it, whereas 'if' would suggest a blanket exclusion of all subsequent delays.
Do they have a general compensation system, or would you also hae to show you suffered a loss as a result of the delay?
AppleMatt