Wow! Hope it works in your favor! You tried to tell them, but sometimes they just don't understand how to fix it, and let it go. I'm sure they have plenty of errors that go the other way, so it balances out.
And while not with Carnival, we had a similar situation with The Container Store. About 20 years ago, we did a 2nd floor addition with all new closets. We were doing the custom Elfa products for the kids closets, which meant having things pre-cut to specific dimensions. So we called in advance (we had 2 year old twins at the time, so didn't want to stand around waiting in the store), and placed our order for like $400 worth of pieces. We gave them our credit card and they charge it all. Once we got there, we had to still go through the checkout line, because we picked up a few small items. They then went through our phone-in order, checking over things, and confirming we had everything that we had charged. I still to this day, have no idea what they did, but for some reason, they issued a refund for the entire order! We noticed it on our credit card statement a month later, but when we reached out, they had no record of the refund on their end! So we did try, but they had no answer either. LOL!