Sadly, those GS bonus items are always an issue, as they have so many employees across different shifts, and it's to much to keep track of. The freebies should have a list of all preorder names on a paper, and attached it to a bag or envelope holding the bonus items. Whenever someone gets an item, their name is crossed off, and you now have a record of where these things went. I learned this selling candy bars in elementary school eons ago.
It's really not a hard thing to do, but someone has to care enough to do it, and considering the GS higher ups are about the biggest crooks I've ever seen, these bonus items are always a 50/50 possibility each time. I only ever preorder when I knew someone at the store, who would make sure I got the item. In my experience, they almost always got more than needed, as I would often get some extras once all preorders had been fulfilled. And for some games, the developers would provide extra posters, etc and allow them to be given out to anyone who asked for promotion purposes.
On a few rare occasions, they would get less than required, as my friend who was a manager would often give up his personal bonus item, to cover a customers order. So it does happen, but more often than not, staff just gives out items to whomever, and don't really care if stuff goes to the right person. Or their taking them to sell on Ebay or whatever else, it just depends.