Agree with the above. We've never ordered an item for which there was an additional charge. The burgers, quesadillas, desserts are included for the basic $15 charge incurred when first using the service. The onboard menu is extremely clear about those items that are extra charge and which are not. Very intuitive so one shouldn' t be concerned about accidentally stumbling into an extra charge item (some are costly).
Pro Tip: The $15 charge is a one-time charge per individual. If two or more are ordering together, e.g. a couple in the same cabin, designate one individual to place all orders rather than each individual ordering separately. In this way the $15 charge appears only once per cabin.
I would certainly not encourage one individual to order for those outside the his/her cabin. I expect that there is some monitoring system set up to flag a large order, or series of large orders, being placed by someone in a cabin for two. If too many try to "game the system", I can see where this would result in some not so nice changes to the program.