It'll come. iTunes bills every week or so. It's a way of reducing their bank and credit card transaction charges. The banks charge for every transaction so Apple leaves it for a week or so before processing purchases in the hope that you'll buy more stuff during that time and they can process a bunch of purchases on an account in one go rather than process each purchase individually and getting hit with higher bank charges.
The transaction charges levied by banks on small payments are a major factor holding back e-commerce.
