sorry for late reply
it costed me total 15k for rear diff oil change, 1.1 litre used, 1 litre new pack cost 10500 and there was another car in for a diff fluid change ( a w212 but with 250k km on the clock), so they only charged me 1.5k for the additional fluid instead of making me buy a bottle as in original estimate it was another 10.5k. labour was 3k. took 20min
though you can get mobil 1 diff oil as well its nearly as good as Mercedes's own provided you can find one that meets the specs. Shahnawaz first undid the fill port instead of drain port as they say often these get jammed and are hard to open, so check before draining. they used a torque wrench to tighten it but i have forgotten the value, could ask them again if you needed. oh diff oil smells really bad. Remember to check the breather on top as well if its in good shape.