Unknown to some trainers, you can actually enter free promos codes for Pokemon Go that will give you free items.
While these aren't common, Niantic sometimes share them on their Twitter account. The Niantic website says the following: "Niantic offers a limited supply of one-time use codes in conjunction with special events or sponsored partnerships."
MDWC4SNGUFXS2SW9 - 20 Great Balls and 20 Razz Berries - expires 8am UTC, April 26
How to enter promo code via Pokemon Go app
Finding where to enter the code isn't as easy as you'd think, at least for some trainers. If you're using an Android mobile device then it is simple, though.
Open the Pokemon Go app, tap the menu button (the PokeBall icon), tap Shop and then swipe to the bottom. There you'll find a space to enter the code and get your items.
Unfortunately iOS doesn't have this option. That means iPhone users will need to redeem any codes through a web browser.
How to enter promo code via web browser
To do so you'll first need to visit the official Niantic Offer Redemption website. You'll be given three options to login with - Google, Facebook or Niantic Kids.
The credentials will be the same as the ones you use to login 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.
However, there isn't an option to login with Pokemon Trainer Club. This means iPhone users who only have a Pokemon Trainer Club credential aren't currently able to redeem any codes.
Fortunately you can link your Pokemon Go account with Google and Facebook and resolve this problem. It is recommended you do so too. If you were to get lose your login or get hacked, both options have two-factor authentication for security.