I did this deal last year and it allowed me to catch up on all the boxes that I missed prior to starting my subscription. The only catch is you have to be currently signed up as a subscriber either bi-monthly or yearly to be able to order the older stuff. Which means you can sign up now for the Jan 2018 box and then go back and order your past boxes. I ended up getting a couple variants in the back boxes that I ordered which was a SWEET deal.
Each box does ship separately and you do have to pay for shipping on each box.