Why am I being charged for free apps?

Did you get an email from Apple about your purchase?

Have a look at this article:

If you see 'apple.com/bill' on your billing statement - Apple Support

This might also be a scam, trying to get your personal data:

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Snapchat may be "free" to download, but it looks like functionality is locked away behind "in-app purchases" that cost real money.