Indygo

Whoa, that’s cruising level 5000 😆

Not a bad plan.  Buy the refreshment package and purchase a shot of liquor on the side with your package included mocktail.  It'll save you around $6 per cocktail.

Ah, good point! That slipped my mind.
There isn’t anything stopping me from taking the non-alcoholic package and then paying for eg cocktails out of pocket?

Nothing stopping you @Indygo

As a reminder, you can bring 2 bottles of wine onboard per cabin.  

Agree with the others, and to address the second part of your question; yes, you can call Royal and have OBC added to your account pre cruise.

You should be checking your bill regularly for errors, so essentially you’ll be price watching.  As speednoodles said, gratuities are included in the drink bill. You can add an additional tip if you choose to.   I’ve never put money on my account up front.  I know the bill can be paid any time you stop by guest services or you can wait until the last night to pay in cash if you didn’t want the credit card bill.   US dollars. 

Gratuities are automatically added to each drink so it'll just all get charged to your on-board account which you'll pay at the end.