The mining process requires energy consumption and human resources for the maintenance. In order to cover these fees all users are issued with maintenance and electricity fees (MEF) accordingly to their contract type and hashrate.
The MEF is linear and equals 0.0035 USD per every 10 GH/s of SHA-256.
Maintenance and electricity fees are not billed in an invoice. Instead the fees are deducted from the customer's balance at the moment of daily payouts. Despite the fact that balance is accounted for in BTC and the fees - in USD, the fees are deducted in BTC according to the current USD/BTC exchange rate which is beneficial for the customer in case the exchange rate trend is positive - more BTC remains in the customer's balance.