Volume and fees are computed on a per order basis, this means that if for instance you create a order to open a position of 10'000 EUR and then a new order to close the whole position, it will appears as 2 orders for a volume of 20'000 EUR.
On the PrePay plan, you would have to pay for each copy of the trade to each of the respective Slave accounts, the cost is 2 EUR for 100'000 EUR equivalent (1 standard lot = 100'000 EUR equivalent), whatever is the traded instrument. 5 EUR maximum per day per Slave. The Prepay plans fees is computed on the Slave(s) volume, not on the Master volume.
The volume is always expressed in EUR and is the notional value of the order. Usually a standard lot is 100'000 units. For instance 2 EUR/USD lots is equivalent to 200'000 EUR. 1 lot of USD/CHF is equivalent to 89'000 EUR (if EUR/USD rate is 1.1235). The trade copier always takes the current market rate to convert the volume in EUR