It seems you have to buy the 5 songs in one transaction, I wonder if this would preclude AU cards.
it could also mean that you'll get a credit for 5 songs even if you don't purchase 5. The T&C seem to imply that as well again:
"use that Card to make
a purchase on iTunes® between 2/10/11 and 3/15/11. A statement credit equivalent to the price of 5 song downloads on iTunes plus sales tax will be issued".
Granted, the heading does say "purchase 5 songs", but that's not necessarily a requirement...