billing fraud?

If 'opened the bill' means you opened an email then it sounds like a phishing email. iTunes would have no need to ask for your payment details for a refund, they already know how the item was paid for and therefore where a potential refund would go. Genuine emails also contain the billing address from the iTunes account. If you entered your iTunes account's password you should change it immediately, and check its purchase history.

Phishing and similar emails : Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Genuine iTunes emails : Identifying legitimate emails from the iTunes Store - Apple Support

That's a phishing email. iTunes knows your name so won't address you as 'dear customer', genuine receipts will also contain your billing address etc. - and the total is wrong (4.99 plus 15.99 is not 29.98).

You've now changed the password ?

I've only heard about it from people clicking on links in phishing emails and therefore going to phishing sites.

Where are you seeing this 'billing fraud' ?