If you get a shipboard credit prior to receiving your documents, it should have the credit showing. If you get a credit after receiving your docs, make sure you get a confirmation of that from your agent.
My Carnival PVP always sends me an email confirmation about any shipboard credits. I print out this confirmation and take it with me on the cruise. That way, should there be any discrepancy or misunderstanding, I have the PROOF right in my hand. (Only happened once...glad I had it.)