When you make a bitcoin transaction, you are sending funds to a bitcoin wallet address controlled by another person (or in our case, BitPay). What the payment recipient receives and then transfers away from the receiving bitcoin address is called an Unspent Transaction Output (UTXO). This is the unit of bitcoin which can actually be used in another bitcoin transaction.
The additional network cost included in your invoice total covers the cost of BitPay's UTXO sweep for your payment.
You can learn more about the reasons for this additional fee here: https://blog.bitpay.com/network-costs/
Didn't I already pay my miner fee?
Remember that this UTXO consolidation cost is not the same as the miner fee included with your initial payment to a bitcoin address. Your own miner fee is important in determining the confirmation time for your payment, which is a function of the Bitcoin network (and not something which BitPay controls).
How can I minimize this cost?
If you're concerned about additional network costs on your BitPay payments, we strongly recommend making bitcoin purchases in larger increments to offset the cost across a larger value.
Any additional payments you have to make to the BitPay invoice will also be charged the network cost. To avoid paying the network cost twice, be sure to pay the exact BTC amount requested on the BitPay invoice. A good way to ensure that you do not overpay or underpay a BitPay invoice is to scan the QR code or pay with a payment-protocol compatible wallet like the BitPay app.