Additional Materials Deadline?

<p>I was waitlisted at Caltech, and I've recently learned that the deadline for submission of additional materials is April 15th. How strict is this deadline? From what I've heard, Caltech doesn't take kids off the waitlist until after May 1st, so would they consider additional material submitted AFTER April 15th? Also, if I wanted to submit an additional recommendation, should I upload it using my Caltech portal or send it to Caltech via mail?</p>

<p>Thanks in advance.</p>