As per the information here:
https://bitpay.com/pay-with-bitcoinThere are TWO "costs" involved with paying by bitpay... Firstly, they have their so-called "network fee"... this fee is what bitpay use to recoup the costs of them having to pass your payment on to the merchant. You're effectively subsidising the miners fee that bitpay need to pay to consolidate your payment into their main wallet/cold storage etc.
This amount is included in the total invoice amount you have to pay bitpay.Payment Amount = X BTC
Network Fee = Y BTC
Bitpay
Total Invoice Amount = X+Y BTC <-- This is the amount that bitpay must receive.
The 2nd "cost" is the miners fee that
you need to pay to send your transaction to bitpay... So, the actual total
cost to you will be the (Bitpay Total Invoice Amt + Miners Fee to send transaction to Bitpay). The miners/transaction fee you pay is dependent on network conditions at the time of payment and how your wallet determines the fees etc.
NOTE: That Electrum will ADD
your transaction fee to the amount you are sending, it does not subtract it... so as long as you have enough funds in the wallet, and you enter the "Bitpay Total" amount into the "Amount" box on the "Send" tab in Electrum, then Bitpay will receive the correct amount.