Retailers are focused on balancing tight inventory management and high levels of customer service.  They want to avoid stock outs so as not to lose sales revenue while also avoiding holding too much inventory that is later heavily discounted to move the excess inventory.   Wal-Mart, who is always searching for opportunities to gain efficiencies in their supply chain, recently announced that US vendors shipping to distribution centers must deliver within a four day window leading up to the must-arrive-by-date (MABD).