If MDZ is your first port of entry into Argentina, you clear immigrations and customs at MDZ both foreigners and Argentina residents or citizens. No way around it. Any subsequent flight from MDZ to any other point in Argentina is a domestic flight which you would need to check-in for after you exit customs. Since MDZ is not setup to collect the fee (like all other entry points into Argentina outside of international flights into AEP or EZE), congrats!