The USPS is a company operated just like a private business, you need to get your information to the top for results. Like many businesses, the USPS has a Board of Governors. Start with writing to:
Mr. James H. Bilbray
Board of Governors
United States Postal Service, Room 10300
475 L'Enfant Plaza SW
Washington, D.C. 20260-1000
Using this same address also write to the following members on the Board of Governors:
Ms. Ellen C. Williams
Mr. Ronald A. Stroman
Ms. Megan J. Brennan
Ms. Megan J. Brennan is not only a member, but the Postmaster General as well.
It may take several letters but you will get good results. If your response letter is from someone other than a Board Member and full of nonsense or doesn't address your concerns, send that letter to the Board of Governors. This does work but does take some time and effort. They do want to hear from you especially in light of the fact that the USPS is going broke.
For Grenada, the scheduled delivery date of 'three to five business days to many major markets' is strictly an estimate. The statement comes with two notations; the dates given are based on average performance history, and actual transit time may vary. This does not take into account weather issues, breakdowns, custom agency delays, and foreign postal issues, all factors which might impact the estimated delivery date.
If the United States Postal Service (USPS) accepted your package and successfully transported it to the country of Grenada, where the package was handed off to the Grenada Customs & Excise Division, then USPS completed the job you paid them for. Based on the description provided with the question, USPS was able to track the package to Grenada. Since delivery of the package would be handled by the Grenada Postal Corporation, is that the agency which contacted your friend and advised him they had the package and would deliver it to him? You were both lucky the package was located and finally delivered.
If USPS never completed transport of the package to the destination country of Grenada, then a complaint could be filed with the Office of Inspector General. You can use their online form, write a letter, or just call the Hot Line toll-free number to make the complaint.
https://www.uspsoig.gov/hotline
"After being able to unsuccessfully deliver the package"? What does that even mean? If the package was undeliverable or delayed due to circumstances beyond USPS control (destination country postal issues, Grenada customs, etc.), that is not grounds for an effective complaint.
sure file a complaint
Of course you can.