The quickest way to send Elder Mack something would be by email. His email address is: firstname.lastname@example.org. He can read and send emails every P-day.
Letters may take 2-3 weeks to get to the mission home and then it still has to be delivered to their area. Postage usually costs $1.15 for a regular sized letter.
Packages may take 6-8 weeks. The best way to mail a package would be using the International padded envelope. It costs $24 for 4 lbs. Larger boxes average about $10/lb.