From the beginning in 1988, I have had nightmares about damaging or losing customers' artwork. So, we have only a few dedicated locations and uniform provisions for storing customers' property. We put it away immediately after taking in the order and leave it in its designated location until we are ready to mount it. Consistency in storage practice is essential.
We use folders (once white matboard, now white Coroplast) to store everything that is flat. The folders are very durable, easy to clean, and easy to handle. When we see one of these distinctive folders, we know it contains a customer's property. We make these folders using a spine of 2" wide packing tape inside and outside the hinge-edge, so there is no chance of exposed adhesive, and we install Mylar windows on an outside corner to hold the order ID tag. We have them in 24x36 and 36x48, and both sizes stack neatly in our ten-drawer, 38x50 steel flat files.
Little items are too easy to misplace, so we make them big. Even if the item to be framed were only a postage stamp, we would secure it in its own 24x36 folder and stack it in the drawer.
Three dimensional objects are kept in boxes on an overhead shelf designated for only that purpose. Flat items larger than 36x48 generally are posters that arrive in tubes, which we use for storage in a designated place off the floor. On the rare occasions when we need to store a large item flat, we put it between two sheets of 40x60 matboard or 48x96 sheets of Coroplast, tape the edges, mark it boldly, and keep it in a designated place.
Our flat files were well used when we purchased them from the surplus office equipment warehouse at Ohio State University. When we need to buy file cabinets of any kind, I check with the university, since they are always remodeling or expanding, and academic folks want everything in their offices to be shiny and new. They have an unlimited supply of money, you know. Anything that looks worn or outdated is replaced, and the old items are often sold for cheap.