You need to order them online. they deliver them to you about a week later.
You don't pay for postage until you get ready to send something, you can use either click-n-ship online to print out postage, or take it up there.
I suggest you get a mixture of boxes, if you are shipping light stuff its often much cheaper to go regular priority rates.
I would get a package of the 6x6x7 boxes, the 12x12x10 boxes, the large flat rate, medium flat rate, and the flat rate bubble mailers
Also there are some DVD/VCR? size boxes that you can get that fit perfectly into the flat rate bubble mailer, I stuff them into the bubble mailer sometimes as added cushion for whatever im sending.