Who made this offer? Is it legit? Did you sign up yourself, or did you agree to someone else?
Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support
Another possibility:
What you saw, may be an “authorization hold”, which is not a fee. After it clears, the hold will be undone or reversed.
About payment card authorization holds from the App Store or iTunes Store - Apple Support
Another possibility:
Check your purchase history to see what you were charged for exactly. All charges will be listed there.
See your purchase history in the App Store or iTunes Store - Apple Support
Another possibility:
The offer is may be for new subscribers only. If you subscribed previously, then you do not get a new trial period.