You'll end up spending an amount of BTC that is equivalent to 90% of the cost of your coffee. So, an example: If you choose the $10 option, and $10 is worth 10mBTC (exchange rate made up to simplify the math), then you'd pay 9mBTC and have $10 available to spend at Starbucks. If you then spend $5 of that at Starbucks, you'll end up receiving a refund of 4.5mBTC (the equivalent of the remaining $5).
You'll end up spending an amount of BTC that is equivalent to 90% of the cost of your coffee. So, an example: If you choose the $10 option, and $10 is worth 10mBTC (exchange rate made up to simplify the math), then you'd pay 9mBTC and have $10 available to spend at Starbucks. If you then spend $5 of that at Starbucks, you'll end up receiving a refund of 4.5mBTC (the equivalent of the remaining $5).