Pre paid gratuities are just that. They are added to the cost of your cruise and when you pay off your cruise, port fees, insurance, fuel etc... and pre paid gratuities then they are aleady paid. Mine never appeared on my sign and sail invoice, not during and not after the cruise. I loved doing it like that.
they can be adjusted either way for good or bad service and/or you can choose to leave your servers a little extra at the end of your cruise as we did.