The final few weeks of training can often be chaotic especially for mail. Once your recruit entered into the final evolution of training they often don't receive mail, and in most cases, won't be forward to their next address.
Once you know your recruit's next address you can send letters and postcards.
Handwritten Letters (USPS): Send your last letter at least 10 days prior to graduation week. (example: Send by Jan 1st for a Jan 15 graduation date) - even sending on Monday it may not be delivered.
Postcards: Your last Monday Delivery postcard needs to be scheduled for a Wednesday delivery (week prior) to graduation week. (example: Send for a Jan 7th delivery for a Jan 15 graduation date)