Why did I get charged for gifting a car bundle marked free in the Fortnite item shop?

If you already own a car bundle (for example from completing quests) that later gets added to the item shop with a new decal, it will be marked as free for you to claim. However, when gifting this bundle, you will still have to pay the V-Bucks price displayed in the gifting tab. 

You can find out more about how gifting in Fortnite works here: How does Gifting work in Fortnite?