Welcome to the Seller Community @naylor!
Once you've completed a transaction, there's not a way to combine it with another already completed transaction. Sorry for any trouble that causes you. If you want the payments to be one, you could refund the original transaction and re-charge your customer for the full amount. Unfortunately that's the only workaround.
Let us know if you have any other questions that come up.
nika
Community Program Manager, Square
