I would try calling (not emailing) the agent. If he still doesn't want to deal with the problem, then you should call RCCL. I imagine you would want to speak to either the scheduler for your job position, or someone in the HR department. The bottom line is if the US Embassy want's an original of the letter, then you have to give them it before they'll issue the visa. The US Embassy is highly unlikely to change its position on this.