How to troubleshoot fulfillment process issues when items are missing or under-shipped in e-commerce?

When items are missing or under-shipped, customer service takes the heat first, but the root cause is usually in the fulfillment process. With multi-SKU orders, freebies, bundles, and split shipments for pre-orders, it's easy for the warehouse to miss items if the picking list isn't clear during busy periods. I used to look at individual complaints, but later realized I needed to categorize them…