I'd do it like this
You (wanting to do the swap) sends the item out (recorded delivery) and when the other person receives it they send you their card (also recorded delivery)
recorded delivery is very important so you can see when the person has actually received the card.
Alternatively, both ppl send cards to a 3rd party who then sends them on. (Still all using registered mail)
On another note: Where are you based? (UK/US/ETC?)
