For iOS Users:
It's a little more of a pain in the butt for you guys unfortunately. To redeem codes on an iOS device, you’ll first need to visit the official Niantic Offer Redemption website found here: (you need an account to see links). You’ll be given three options to log in with – Google, Facebook, or Niantic Kids.
The credentials will be the same as those used to log in to the Pokemon Go app. The next page to load will allow you to enter the promo code, and if accepted, a message will display telling you the items have been added to your inventory.
As for the Hat and T-Shirt that you can acquire, this will unfortunately require you to be able to spoof to a location to do so (easy enough if you are a PGSharp user) or be close to an official Fendi sponsored stop.
Good luck and feel free to add to this thread whenever you guys find useable codes (Not 100% sure when these expire, so be sure to grab them while you can )
For reference, here is a list of expired codes from the past year: