As anyone who has ever bought something from the iTunes Store has probably noticed, Apple doesn't email you the receipt for your purchases immediately after your transaction is complete. In fact, if you've ever looked closely at your bank statement for your iTunes purchases, you may have noticed that the cost of your purchase isn't actually charged to your account for some period of time - a couple of hours to a couple of days - after your purchase is complete.
A store not actually taking your money at the time it gives you your purchase is a little bit unusual. So what gives: why the delay in iTunes Store billing?
The answer, it turns out, has to do with user behavior, credit card fees, and trying to drive additional sales.
Most credit card processors charge their clients both a per-transaction (or sometimes monthly) fee and a percentage of the purchase they're processing. On a big ticket item - say an iPhone or a new laptop - the retailer can absorb these fees without much trouble. But, for a very small payment - say US$0.69, $0.99, or $1.29, the cost of individual songs at iTunes - it becomes harder to make money if you charge a credit card each time someone buys a single song. If you were to do that, the iTunes Store would drown in a sea of fees and one-off charges, making profitability all the more difficult.
However, user behavior is such that when you buy a single song or album from the iTunes Store, it's likely that you're going to buy another - often pretty soon after. And that's why Apple delays its billing. It's betting that users will make additional purchases at the iTunes Store and then instead of charging $0.99 to your credit card, they can charge, for instance, $10.98 (the cost of the one song you bought one day and the album you bought the next day), which is an amount easier to derive profit from.
There's also a second, more subtle, aspect of user behavior at play here--at least according to a Wired article. This article, about the ways that companies use to get consumers to act how they want, suggests that by charging you hours or days after you make your purchase, the acts of buying and paying and broken up into separate things, rather than feeling like part of a single process. In that way, the buying can almost seem free--and who doesn't like getting something for nothing (or at least feeling like they are)?
These techniques don't always work, of course - lots of people don't make numerous purchases spread out over a few days or are keenly aware of what they're spending - but apparently they work often enough for the strategy to be effective for Apple.