I want to be the couple who has friends/family who will give $40K at an engagement party! Wowza!!
Most halls will tell you to keep the card box away from the door - or a family member should mention it. Also, don't have a card box that can easily be picked up and moved. We used a lawn fountain (wooden wishing well) for our card box. That thing was so cumbersome but everyone would have noticed if someone was moving it.