You shouldn't run up massive bills.
Bart is shocked at the ability to buy songs for 99 cents.
Bart wants to buy some songs.
Bart decides that even 99 cents can add up, so he limits how many songs he can buy.
Bart gets a bill for $1200 because he bought so many songs.
Bart avoids ever getting a massive bill for downloading songs.
Bart gets grounded until his allowance pays off the $1200.