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.
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.