Detecting and reporting incorrect FBA fees both quickly and accurately is a surprisingly non-trivial task in the Amazon world.
 We opted for having two separate checks in place working together.
You'll find both of these alert messages in the FBA Fee Alert section.

Alert #1 - FBA Fees calculated from Dimensions and Weight

We use Amazon's official formulas to calculate your FBA Fees. If we detect size or weight change in your product details your FBA Fees gets reevaluated immediately. If you then belong to a higher tier, potentially by an incorrect measurement or error, we alert you straight away so you can look into the issue.
This alert is very quick. It allows us to report a problem potentially even before a sale has occurred with the wrong FBA fee.

Alert #2 - FBA Fees extracted from Order Data

 In certain cases, Amazon charges a different fee from what was generally expected. We have also seen cases where different SKUs get treated differently. To catch these problems we check the exact fee breakdown of every order and compare it against the supposed FBA Fee value. If there is a mismatch you get alerted.
Caviat: This alert is slow! Amazon only releases the full fee breakdown of an order two days after the sale. However, this is the best way to track the actual fees charged for sure.