I would just add - though this is probably obvious, but still - WRITE. In handwriting. If your handwriting is bad, you can print the letter, but sign it in handwriting, maybe write his name on top and a line or two in handwriting. Plus, handwriting on the postcard. I would do this (just compiling the suggestions you have received so far into one):

- Mail a standard large envelope to his office with "personal and confidential" printed on it. Hopefully he will open it automatically as he opens all his office mail, both outside and interoffice.

- Inside the large envelope place a card. Either make one by hand or choose a really nice one with a message that is neither corny nor trite. You are in the UK so I do not know what is available; in the US, Papyrus greeting cards are really good - tasteful, whimsical, touching.

- Inside the card, write, by hand, that you are inviting him to (see hannabee's suggestion)

- Also place a small sealed envelope with your printed letter inside the big envelope. On the envelope itself, write "a few things I needed to say".