Buy a Starbucks eGift Card worth $10, select Visa Checkout as your payment method, and you’ll receive a $15 gift card instead.
The promotion does not appear anywhere during the ordering process, and there is no confirmation of the bonus, both of which I find annoying. Also, the terms state it will only last for the first 200,000 purchases, so we don’t know exactly when this deal will end. With that said, I just did it myself and received a $15 gift card, as expected. As long as you get in on this relatively quickly, it should work for you too.
Limit is one per person. Don’t try to game this, as they do track your address, IP, credit card, etc.