Mandy, That is not a stupid question. We don't have stupid questions on here.
I read and then re-read your question, and then went to the Carnival site to see if I could help.
You can only buy wine on the Carnival site, and in order to buy alcohol in the ship stores, you have to let them hold it for you until the last night, or even the morning, you get back into your home port. They will deliver it to your cabin.
You won't be able to find Pepsi anywhere on the ship, because Carnival has a Coke contract. I have never seen any Coke product sold by the liter.
Hope this helps