Some things to note:
1) I went to several BCP agents to try to get the money order, and even showed them the requirement on the Peruvian Goverment's website to go to any BCP agent and get it. None of them understood, and persisted in calling it a transferencia, and insisted that they could only do them for clients. At the third one I went to I complained and showed them evidence that people had done it before (from this page). The gentleman there finalyl became helpful and told me my best best was to go to the principle agency for Arequipa, which is across the street from RENIEC in el centro. I walked in there and went to the tellers' line. I told them I needed a "giro miami", and their supervisor immediately piped up and said "for the US treasury, right?" well he said it in Spanish, but there at least they knew exactly what to do, and I had it done in about 10 minutes. I highly suggest going to the principle agency of BCP to get this done smoothly and efficiently without any hassles.
2) I am doing my papers on the basis of marriage so I needed to have my wife's DNI and a copy with me. I was worried because it hasn't been changed yet to show our address in Peru or her civil status as married. The officer (Ali) at Interpol let me know that it was no problem that it wasn't updated yet, as it is only Migraciones that needs it correctly. He was very helpful.
3) I don't have antecedentes from my home country (USA), but I was arrested in another country once and tried and given a suspended sentence. I wasn't a resident of that country, ever (I was on a temporary visa), so I had no plan to turn in a police clearance from them. However, this came up in Interpol's system, and I was cautioned that I needed to turn this in to Migraciones as well, or perhaps that Interpol already had entered it into the Peruvian system (he was talking very rapidly). He suggested it was unlikely migraciones would issue me the carnet, but that it was up to Migraciones ultimately.