Send your slides and photos as they are, in boxes or carousels, or you can use some of these ideas.
Believe it or not, aluminum foil type boxes make great temporary slide containers. You can also make the beginnings of slide groupings (50+ slides) by using mini post-its to label where they start. Make sure not to touch the sticky part of the post-it to the film.
If the slides do not fill up the entire box, cut a piece of cardboard into the size of a slide and put it after the last slide to protect the film. Then, wad up tissue paper or plastic bags to fill the space to the end of the box.
Rubber band the outside of the box well, so the slides cannot fall out or become mixed up if the box is turned over in shipping.