Welcome to iMore!! If I remember correctly, there is no charge for the app itself so your billing issue is with Experian not Apple. I suggest contacting Experian to get that taken care of.
It appears they have a couple apps depending on what other product or service you have from them. If you have the app just called Experian. While it IS free to download it appears to offer in app purchases, perhaps credit monitoring service, or a credit report dispute? Anyways THAT is what you ordered it appears. You should contact them directly here on their web form...