Weighing in with an official response from the USPS. I work in Trade Policy at USPS. I only noticed this thread while searching for something else, and I do see that this thread is rather old now, but want to weigh in just in case someone might find this useful.
The "Freely Associated States" - such as Marshall Island. Are "treated" as domestic locations where rate is concerned, addressing, etc. The ZIP Code is a US Domestic ZIP for Freely Associated States. While, yes it is an independent nation-state, it has negotiated (with the US Department of State) a Compact which allows it to benefit from the association and postal services of the US.
When sending to any Freely Associated State, or Territory or Possession of the US (except Puerto Rico, which is actually treated as a full part of the US, and "lives" inside the Customs Territory of the US) from any location that is inside the Customs Territory of the US - CTUS - a customs form is required. Customs forms are used both to and from all locations outside the CTUS (including all Military and Diplomatic Postal addresses) when sending to another such location, or to any location inside the CTUS (there are some exceptions within island nations wherein they do not require customs forms when sending between Freely Associated States).
All of this is in the USPS Domestic Mail Manual, and I've always found it useful to "google" such things with the letters DMM and USPS in the search.
These locations - all PTFAS (Possessions, Territories and Freely Associated States) are DOMESTIC rate locations, NOT International Rate. Confusingly there are sometimes similarly named nation-states in some areas that are not part of the Freely Associated State compacts, but being a separate nation-state with political independence does not itself mean that a location is not part of a compact with the US to become a FAS. If you see a ZIP Code associated with a location (in a Wikipedia page, or a USPS site, or other source of information) then it's a good clue that this is part of an agreement wherein this may be a domestic rate location.
Now, whether that location has its own Customs Agency (like Guam does, or other island nation), is not part of the "rate" discussion... and is a separate topic to review as this is entirely governed by the destination's political sovereignty/ laws. So, USPS is not assessing any such fees/ tariffs when something is going to a nation-state that levies such fees, we only do our best to report such things in the Individual Country Listings of the IMM (International Mail Manual) or appropriate country/Freely Associated State information within the DMM (Domestic Mail Manual).
