How can I best book the money I use to top up my prepaid credit card?
If you have purchased a prepaid master card, you must top it up in order to pay for online purchases. Since you have not yet "spent" the money and therefore do not have an invoice to enter, there is also no expense.
Ultimately, it is about the expenses that you actually make. You do not have to state the upgrades in your administration. However, for the Income Tax Return we do need to know the balance on December 31st.