Multiple transactions for a single payment in bitpay
I contacted BitPay support about the issue and they said that it'll just work.

So we made our payment via 3 different transactions. As soon as all transactions were made, the BitPay window informed us:"This invoice has been paid." At that moment either one of the transactions had a single confirmation or none had any. Then came the confirmations.

Addendum - The official response from BitPay support for question on paying via multiple transactions:


  "That should be fine. However with bitcoin you can structure a transaction to be paid
  from several accounts at one time. That's one of the cool concepts of
  Bitcoin. I recommend you check it out on wikipedia.
  
  For sake of time your method should be sufficient. Make sure you do
  sufficient fees to speed up the transaction."