This, pretty much. It sucks that DHL messed this up, but the rep is not the one who did. However, they can help you figure this out. If they reply nicely they're much more tolerant than I. I imagine that taking a lot a people's crap is part of the job, but I would have just deleted the message.
The customer service representative that reads your email is in no way responsible for the delayed invoice you had received. I prefer being polite.