When the item is in your cart, you should check the box that says "I'm buying this game as a gift for someone else" and you will receive a gift code (if I remember correctly) or the ability to print a gift card to give your friend once the order is completed.
If you already completed the purchase (and it looks like you have), you can always e-mail Telltale at
support@telltalegames.com but I'm pretty sure the solution they usually advised in the past (in cases like yours or when someone wanted to add or remove an item from their order) was to have them cancel your order and have you place a new one.
Today being Saturday, I don't think you will have much luck contacting Telltale but you can always send the e-mail today, place a new order and they shouldn't have any problem canceling your original order when the weekend is over.
Remember however that I'm not in any way affiliated with Telltale (just a regular customer and and avid "forum peruser") and that this is not an "official" Telltale answer.