If you want to use your onboard credit to buy the soda package, that’s okay, as the price is the same whenever you buy it. You can only use your onboard credit once you get on board the ship, meaning that your drink package will cost an extra $5 per day than if you pre-purchased it. You can buy a Carnival drink package with onboard credit, but it’s not recommended. Can you buy a Carnival drink package with onboard credit?