I would not be confident in postal systems routing letters based on ISO country codes. If you send your letter in Tennessee with AR at the bottom of it, it's going to Arkansas or nowhere, not Argentina.
Yes, there are actually standard postal country names assigned jointly by the country concerned and UPU (the organisation that manages how mail works globally). For example, the correct country name for ISO GB is "UNITED KINGDOM" (https://www.upu.int/UPU/media/upu/PostalEntitiesFiles/addres...)
Pedantic: actually, there are (up to) two valid postal country names: the English one (UNITED KINGDOM) and the French one (ROYAUME-UNI, https://www.upu.int/UPU/media/upu/PostalEntitiesFiles/addres...). This is due to historical reasons: (jointly) Britain and the US (which use English) were the most advanced in postal services but the diplomatic language at the time is French.
That always trips a lot of people up because .uk is the country-code top-level domain for Great Britain, and cctlds are assigned using the 2-letter ISO country code for that country.
...except for Great Britain, who were one of the first few other countries to be connected to the internet, and decided to use .uk before people realised that every country would need one, and using already-assigned ISO codes would be the best way to hand them out.
Partially this is because the Joint Academic NETwork (JANET) in the uk used addresses in the form of xyz@uk.ac.qmc.cs according to wikipedia the JANET NRS had uk as the top level prior to dns adopting ISO codes in 1984
https://en.wikipedia.org/wiki/JANET_NRS
Huh? The country is called "United Kingdom of Great Britain and Northern Ireland", or the UK for short. Strictly speaking, if you talk about Great Britain, you're excluding Northern Ireland, but politically GB isn't a country...
Wikipedia says "Great Britain is an island in the North Atlantic Ocean", although historically there was also the Kingdom of Great Britain.
Letters like K for kingdom are generally avoided in codes, since this way a change of constitution to the Republic of Great Britain and Northern Ireland doesn't require a new code.
Yeah, GP is shooting the messenger. If the GP is reading this, here is the address of the ISO 3166 Maintenance Agency:
ISO 3166-1:2020: Codes for the representation of names of countries and their subdivisions — Part 1: Country code
ISO 3166-2:2020: Codes for the representation of names of countries and their subdivisions — Part 2: Country subdivision code
ISO 3166-3:2020: Codes for the representation of names of countries and their subdivisions — Part 3: Code for formerly used names of countries
Maintenance Agency
ISO 3166 Maintenance Agency
c/o ISO Central Secretariat
BIBC II
Chemin de Blandonnet 8
1214 Vernier
Switzerland
Tel: +41 22 749 01 11
E-mail: customerservice@iso.org
Website: www.iso.org/mara/iso3166