There are two types of In-Game Items, both of which are purchased differently.
1. How to purchase in the Microsoft Store (NP & DLC)
Go to [Microsoft Store] - [Eternal Return Store Page] - [Add-ons for this Game]
After purchasing an DLC product please collect it in the in-game store menu.
2. How to purchase In-Game (NP Only)
(In game purchases are currently available. To purchase NP, please go through the Microsoft Store)
In the game go to [Shop] - [NP] and select the amount that you want purchase.
A pop-up will appear taking you to the Microsoft Store. Please login to your Microsoft Account.
Follow the purchase steps to complete your purchase.