Why am I being charged 7.30$ for roblox?

Then there may be an issue with the payment method you are using with your Apple ID. Even if the app you are trying to download is free, a valid payment method is still required.

Ref:

• Payment methods you can use with your Apple ID - Apple Support
• Change, add, or remove your Apple ID payment methods - Apple Support

Alternately, you can contact Apple Support directly. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.

Did you get this app directly from the Apple App Store? Where are you seeing this charge?