I don’t understand the point of the question.
People ship goods all the time with packing lists and bills of lading. This seems to be essentially what you’re doing here: sending a package of documents. So include a document index that the person must acknowledge and return in order to complete delivery.
Alternatively, the documents can be sent individually via certified mail, return receipt required, with each document holder (envelope) clearly marked as to content.
Of course you can’t prove what the letter “says”. The ‘proof’ of a letter is the content itself. Even if you can prove what it ‘says’, you can’t possibly prove what it ‘means’. This discussion is turning away from a practical means to send and verify specific content and more into epistemology and meaning.
What is so all-fired special about this letter / document, over the hundreds of millions of other pieces of mail that run through the US Postal Service every day of the week?