Thanks Saka and thanks guys for the support and advice. The passports have just been sent, I didn't want to wait until Saturday. I secured the two passports with a rubber band and placed them in a large brown envelope and placed that inside an Xpresspost USA envelope. I was originally going to send it just by regular mail but I didn't want anyone else seeing the File Number on the envelope so I decided to keep that secured inside the Xpresspost.
The only thing with Seattle is that the only commercial courier mail that they return mails with is FedEx but I didn't have an account opened with FedEx so it's just going to be sent back to me by First Class US mail. So that's my current update about the PPR, and I hope I can just wait for the four weeks without any worries.