Each station receives two more packs. Have add_delivery visit every element and add extra directly to it. The print in main reads the original array after the call, so its values will show whether your changes reached the caller.
void add_delivery(int stations[], size_t count, int extra) above main. Add extra to each of the count elements. Keep the supplied call so the program prints Stations: 5, 6, 7 with a newline.Loop while i < count, adding extra to stations[i] each time.