In an order I had submitted a few months ago, I had accidentally entered "Quantity: 1" for my 1st Edition Squirtle, even though I was submitting 2, along with 100+ others.
CGC was awesome and sent an email within a few days of the submission being received saying that there was an error on my end and they updated the submission to show +1 cards, and added the same declared value for both of these cards. I had to do nothing, and it was handled seamlessly on their end. I truly didn't even check if I was charged the $2 fee or not...