In-game items and currency can’t be transferred between different accounts, so it’s important to always double-check your Customer ID before making a purchase.
However, if you play on multiple devices, you’re all set, as long as your game progress is synced to the same Customer ID. When your account is properly synced, you’ll have access to your purchases (and game progress) across all your devices.
Just remember: the key is using one synced account on multiple devices, not separate accounts for each device.