Well for one thing, M-B dealers typically don't negotiate. And even when they do, their cost for the parts normally exceeds the online retail price. The Parts Manager that I was dealing with told me that he doesn't have a 100% markup on air filters, so that means his cost couldn't have been lower than $20. That already exceeds what I paid for the filter online. MBUSA which sells them the parts marks them up heavily before the dealers do their own markup, resulting in stratopheric prices that I definitely won't pay.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver