Not sure about the current situation but the customs office was on the left side of the KLM departure hall (which is to the left of that for other European carriers) as you are looking at the terminal from the drop off area.. I just don't remember what one does with the bags.

Just be aware that if you are seeking to get cash back on the other side, the lines can get pretty long. Last time I did it, I was only getting EUR 5 back while the mainland Chinese tourists in line were getting what were 100s of Euros.