[berpasan]

We needed this letter for a founder who didn't have a VISA before we're invited...

It's very simple and brief, one page, just says my co-founder was invited to an interview for funding, says when the interview will take place, says the request is urgent and explains what YC does and has the partner contact info for questions.

Hope you are not trying to forge one! They will be really quick to issue one if you need it (for legit reasons).