Witam i pozdrawiam!...
An interesting question, I've pondered that problem many times. A word being a very close equivalent of the Polish 'pismo', even though not exactly meeting your very detailed definition ( ) is 'correspondence' used as a broad, general term in similar contexts. For example
"...in our most recent correspondence we pointed out that..."
See what I mean? It works for a posted letter, fax, even e-mail...
In some specific contexts the word 'message' can also be used.
Hope the above helps a little bit, at least.
All the best,