1. Once you click the buy button, a payment window will popup prompting you to enter an email ID
2. Once you click confirm, a new window will appear prompting you to select payment method
3. The third window will ask you to enter payment details (such as credit card number etc). There will be a "Have a promocode?" prompt. When users click on that, it will give you a tab to enter the old CD Key and apply the discount

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