Does that mean you have to look up each item's weight & dimensions on Amazon.com? YES!
Does this mean I could actually lose money selling a book? YES!
See the chart for cost comparisons. Keep in mind, while this happens a lot, the more common error is one that is off by several ounces, not several pounds ... an error that you would probably never notice.
Also keep in mind, it does not take an error like this to lose money on an FBA transaction. Sellers lose money all the time - by being forced to lower prices, by having an item sit in storage too long, on returned merchandise, by FBA restocking fees, etc:

