After you make the payment, you will receive a key code
only one code can be used per account

1. Login to the Battle.net Desktop App
2. Click your profile icon at the top right corner of the App
3. Click Redeem Code
4. You can also redeem at https://shop.battle.net/ by navigating to Redeem Code under Account.

Note: Codes must be redeemed on Battle.net specifically, but will appear for any platform where that user has a license.