Yeah, paying by Apple credit account for a $300 device smacks of poor financial planning to me. If you don't have $300 handy, or a regular credit card to put it on, then save up for a while until you do.
A common trap is to think to yourself "Oh, I'll have it all paid off in just a few more weeks so it's not a big deal". Would be true, except that a few weeks tends to turn into a few months, and in the meantime Apple releases a new Touch or tablet computer or iPhone II or something, and since you allowed yourself to do it once, you figure it wouldn't hurt to put that one on credit too, and suddenly your $300 turns into $3,000 and takes years to pay off. Not to mention all the interest you're paying on it.
It sounds ludicrous when you read it on paper like that but how many people do we all know who've been there, done that? Myself included. My little 12" PowerBook G4 probably cost me on the order of $5,000 if you include the three years of debt and interest payments that it threw me into.