On the Mercedes service website, when it gives you the price click on
+ Additional Items to see any items which are due at your car’s age and/or mileage (see photo below as displayed on a phone).
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After clicking you’ll see ma breakdown of the additional items and the price of each - these are all included in the price you saw on the previous page, so the price quoted if the price to do everything which is due.
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If the transmission fluid/service is due then it would be listed as an additional item. IMPORTANT: Please note that if an additional item was skipped during previous services then the online quote tool won’t know that and won’t say that it’s over due.