Confirm what the fee was applied to
A percentage without a base is ambiguous. Check whether the applicable agreement uses a selling value, a value after a particular adjustment, or a category-specific price basis. Do not assume every marketplace uses the same definition of NSV.
Check the rule selection
Review the relevant category, price slab, quantity, zone and effective contract period. An unmapped or wrongly mapped category can produce an apparent overcharge even when the marketplace charge is consistent with its rule.
Handle signs and reversals
A refund may reverse some components and introduce a return fee. Compare equivalent lifecycle events. A negative commission on a return is not automatically an error, and a cumulative report may contain both positive and negative legs.
Prepare the evidence packet
Include the source order line, settlement entries, relevant agreement clause, expected component calculation and specific difference. Describe the discrepancy precisely. Retain the marketplace acknowledgement and update the case when a response or payment arrives.