There are different coefficients and cost for copy trading depending on the balance of the wallet connected to a strategy and the copy trading subscription cost. During the active paid period if your balance increases and reaches a certain balance range of the higher coefficient, you will receive the extra invoice to pay.
The amount depends on several criteria:
the exchange balance at the time of subscription activation;
the coefficient balance range you have reached;
the date when the balance was increased;
the paid subscription dates;
the cost of copy trading (set up by the strategy trader).
You pay for it at the end of subscription.
There are two pages where you can see how much you've paid and for which time period:
Please read more about the extra payments and how they're calculated in the article.