Sure, just add 15 euros to your existing pledge via the Manage Pledge button. For example, if you have pledged 16 euros for the Early Bird Digital, set your pledge to 16+15=31 euros via the Manage Pledge button. Your reward tier will remain the same (Early Bird Digital), you shouldn't change that. If in doubt, send us a message and we will check if you have set it properly.
Yes, PayPal will be open from the last day of the Kickstarter campaign. That day, we will update the Kickstarter page and our webpage with more information. PayPal is preferred to bank transfer, but we received a few requests to accept bank transfers as well. This is not always possible, so if you cannot use PayPal and want to use bank transfer, please send us a private message.