Your checkout page looks nice and simple. I only have a few tips: a) you can emphasize the total price (by perhaps increasing font or having it in a different color). Also, research has shown that #1 reason of cart abandonment is extra shipping costs. Visitors generally prefer to pay only what they think products costs. So, perhaps you can make that clear on the product pages itself (to set the right expectation). Also, perhaps you can minimize 'Coupon Code' tab and 'Newsletter' tab by default as it is not relevant to checkout.