I understand you can send mail certified so you have proof of mailing something, but is there any way to certify/legally prove the contents?
Depends on what the contents are. You may need documentation from an expert in that field. Or some things can be notarized.
I have had to do just that with info sent to the government etc. I had to get the letter/form whatever signed by a JP (justice of the peace). T find one I only had to look in the phone book.