I forgot to mark how many liters I fuelled through the app.

No worries if you forgot to add the liters you fueled in the app, Skipperi's customer service can add the fueling afterwards under that boat reservation. So please send a message to the customer service with a picture of the receipt which clearly states, when, where and how much you fueled and for which price,  in addition to information about the boat reservation and we can correct the balance.