Shipping cost in most ecommerce backends is an estimate that nobody checks. At checkout you show a number from a rate table. The parcel ships. The carrier sends an invoice six weeks later with a different number on it. Nobody joins the two, because joining them means matching a carrier's line-item PDF against your order ids, and so the difference just disappears into cost of goods sold and the margin dashboard keeps lying. The fix is unglamorous and it pays for itself: a ledger that record...