Walmart expects you to cancel an order anytime the customer has requested cancel up until you mark it as shipped. If you were using the walmart system exclusively you would see warning flags and would be unable to easily print postage. If you are printing packing slips from 3D and using something like ShipRush or Stamps to print your postage you would have no idea you were not supposed to ship the order. In most cases you just cancel the order, when you notice the problem, after you have shipped it and take the loss.