Look closely at the T&Cs.
"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 generally within 5 business days after your qualifying purchase"
Looks to me like Amex will credit you for 5 songs +tax based on simply a "purchase", i.e. one .99 cent purchase gets you $4.95. YMMV
Andyandy