I think you can order up to 4 items a month without further shipping. Only had problems with added customs as an Intel because they declare correctly. If you buy extras as an society member they have a nice drag and drop menu, where you can decide to which months you want to add the extras.
I swapped this month. They just told me that older society titles are game for swapping. You just need to email.
If you're predominantly looking for older titles you might want to wait till November. They had great black Friday deals on memberships last year. I joint for an International yearly than because it was like 25 or 30% discounted