If you've bought the standard subscription and then decided to switch from standard to premium, you can pay for the margin only if you pay for the subscription via the iOS app.
When you purchase it, you'll have yet another subscription that will function together with your standard subscription, upgrading it to premium.
If you've bought the subscription via the Android app or at the website, there is no way to pay only for the extra charge.
In order to read premium books you'll have to buy the premium subscription in addition to the standard one. To buy the premium subscription you'll have to cancel your standard subscription first and then purchase the premium one. Don't worry that you've paid for the standard subscription in vain. Contact us and we'll recalculate it to premium and add some extra days to your subscription.