I know there is a department at the Carnival offices in Miami that you can speak to directly. I believe it is Guest Relations. As long as you have all the paperwork, receipts, etc., to send you should have no problem.
Carnival gathers this information and if there are numerous complaints with one particular vendor, they will no longer use them and send passengers to their store.
See if that helps, and keep me posted on this thread. Hope I was able to help.