There is only one per case, so they go fast. But, you can very nicely ask the people at Target if they have anymore HW cases in the back, and if they do, they will bring one out for you to look at and go through.
When they do bring out a case for me to look at, I will offer to peg the cars for them as I go through the case. And, sometimes I ask to keep the box so I can haul off my loot and store more of my collection in them.
Target folks are usually pretty good about it, just be nice and don't leave a mess and they will be nice to you when you come back. And Walmart pegs their stuff at night and they usually don't have anything in the back.