I was concerned about sending documents to my fiancée recently. I'd read on here about the dangers of sending with USPS versus some of the other services. However, when the time came for me to send the documents, after receiving the NOA2, I started looking at pricing. For 6 1/2 ounces of envelope and papers (about 30 pages, give or take), UPS wanted $90, FedEx wanted $80, and the USPS wanted $45. All services said roughly 5 days delivery, with FedEx offering 3 day for a bit more. I figured with about 3 weeks available I would try my luck with the USPS. I had extra copies of everything, including original certified copies of my birth certificate and divorce decree, so I certainly could have resent via FedEx later, if documents didn't arrive.
I sent the middle of last week, Wednesday April 24. I hoped for arrival by the end of this week and planned to send via FedEx this Friday if USPS didn't make it.
Long story short, the package was delivered Tuesday Philippines time, just 6 (5?) days after I sent the package. No muss, no fuss, no bother.
I can't say it would work every time, but in my case is was easy peasy, and I would recommend it.
Good luck to all.