I'm pleased to announce that I recently published a new official Phaser 3 tutorial all about adding In App Purchases to your Facebook Instant Games. It covers the whole process, from creating the product catalog, to display it in-game, making purchases, rewarding the players and how to test it all without spending actual money.
If you're considering using In App purchases in your game it's well worth a read, especially as your game goes through a more stringent approval process by Facebook, so you want to get it right first time.