Simple past is the most appropriate tense here. This is because, in this case, you're just referring to something in the past: You sent your person. The office was closed. You don't need anything more complicated than that.
Past perfect is used when we're referring to something that occurred before something else. For example, you might say, "Before my person returned, I had thought that the office was open," because the thinking occurred before your person returned, or "I had sent my person to the office before I learned it was closed", because the sending came first, then the learning. If you say "I had sent my person to your office but it was closed", it's unclear, because it sounds like you're going to refer to a second event, but there is no second event in the sentence.