Unfortunately your situation is exactly why I always ask for the Guest Copy from X. Any chance you have yours, not just the invoice? It will contain two line items for each item included in your fare. This is a copy & paste from our Guest Copy for an upcoming cruise. The first line item shows the billed amount for the item and the next line shows a credit for that amount. I have no idea why "Gratuities" isn't in some cryptic code like all the other line items.
Gratuities
TCI9-GRAT DLX 7N
TCO4-BEVPRINGRAT7
TCO3-PROMO2P 7N
YVY2-INTERNETCHG
YVY3-INTERNETSAV
I know on previous cruises the daily gratuity would show up on the account but it would always net out so that at the very end the only balance left would be for any souvenirs we purchased. If you don't have it, I would see if you could get it from X. Without it I'm not sure how successful you'll be arguing it with X or even trying the chargeback route with your CC company.
Best of luck.