I don't blame you for feeling that way.
This is what I have done, and never had any problems.
I use to take the "dollars" with me, and had them applied.
I now order them on line, and have them applied from the computer.
This is how I do it.
Order from the web site.
Instead of putting your address in the spaces provided, put your cabin room number on one line, and on the next line, put the words, ON BOARD CREDIT.
I have never had a problem, and have in the last couple of cruises had my on board credit paid for, before I ever cruise.
If for some reason you do not use all, they will cut you a check on the last night of the cruise, and even cash it for you, before you leave.
What you are trying to do, is common sense, what they are saying to do, makes me do this.