When purchasing Bundles and Ancient Coin packs for Sea of Thieves, it is important that you purchase through the correct store for the platform in which you are playing. Otherwise you may not receive the items in which you have purchased!
So this means that if you are playing on the Steam version of Sea of Thieves, then you must purchase your content through the Steam store. And likewise, if you are using the Microsoft Store or Xbox version of Sea of thieves, then you must purchase your content through the Microsoft Store.
This is due to how your purchases are redeemed into your game. For example, purchases through the Steam Store are added to your Steam Inventory, which in-turn are 'consumed' into your game during the next boot flow with the item then being removed from your Steam Inventory (the Microsoft Store / Xbox versions are not able to process these purchases). Likewise with the Microsoft Store / Xbox Versions, purchases through the Microsoft Store are not added to your Steam Inventory and therefore cannot be 'Consumed' into the Steam version of the game.
If you do mistakenly purchase Bundles or Ancient Coin packs on the incorrect platform store, please contact the following support channels for a refund.