The letter (which I presume must be some kind of season ticket holder gift, given the way it's written and the branding) might be legit, but I wouldn't put any stock in that ball, particularly with a third-party COA. If it was a legit Knicks item, it would be cert'd by the team/management instead of Online Authentics.
Running a search on the ID pulls up a vague listing with no signing dates, and only some general "physical review" info.