How this function works. When you enter the merchandise in Fortnite and select an item you want to buy, you will now be presented with two options: buy it yourself or "buy as a gift". Select the latter and Fortnite will provide another screen where you can choose a friend to send the item to. You will be able to add a custom message if you want. Then send it off and your friend will receive a gift box with the item inside the next time they burned up the game.
It is worth noting that you can not give gifts to anyone. Epic Games has set up a system of requirements designed to prevent people from hijacking an account, buying a lot of stuff and sending them to their other account. First, you must have multifactor authentication enabled on your account to send gifts. You can also just send gifts to people you've had friends with for at least 48 hours. Finally, you can only give three gifts within a 24-hour period.
When you decide to give a gift, the purchase is non-refundable and you will only be able to provide gift items that are actively available in the store. If you are more than a recipient, you can turn off the ability to get gifts in your account settings.