Every plan in AdNabu has two aspects. There is a fixed price and a percentage amount. Lets consider the Basic Plan for the scope of this question.
In the Basic Plan, the fixed price is $99 while the percentage value is 7%. This means at the end of every month we will consider both the amounts - $99 and 7% of your spends and charge the maximum amongst the two. This essentially means you will be charged a minimum of $99 if you have purchased the Basic Plan. Please note $99 will be charged at the starting of every month. Any additional charges will be levied in the next invoice. Lets take an example.
Lets say the Adwords spends of the current month is $600. 7% of this value is $42. This is less than $99. Since you had been already charged $99 at the starting of the month, you would not incur any additional charges in the next month.
However if your spends were $1700, considering 7% of spends, the amount will be $119 which is greater than $99. Since $99 was already charged at the starting, the remaining amount which is $20 will be charged along with the next month's invoice.
PS: Only campaigns created via AdNabu will be considered while computing the charges.