[fabricode]

Everyone seems to be confused by the "I am sending you the only copy" aspect of this letter. I'm sure he kept excellent records, and keeping a copy (or the original) for himself would be a standard business practice for someone in the communication business.

It is no different than you sending an email to someone saying, "I'm sending this note to you alone rather than posting this publicly, ..."

As for why he would keep a copy: no different than why you keep copies of your emails. Should the recipient respond, he'll have his original to reference if the person responding takes items out of context, attributes statements not actually made to Vonnegut, amongst other benign or nefarious mistakes.