Like a lot in life, you can try to control a situation or just let it control you...if you use a Merc dealer:
The standard dealer approach is to say "we can't give you a price until we hook the car up - it tells us what needs doing".
You then get a low(ish) price for the oil change and quick look (a B service is an A service with a bigger quick look involving the wheels coming off). Cut £60-80 net off this by supplying your own oil from Costco.
On the day, you get a call listing two sets of extras:
a) mileage related (air filter, fuel filter, plugs, ATF).
for these, you get quoted book price for labour as if JUST that job was being done - i.e. paying again for the time to put the car on the lift, connect to STAR, take the wheels off etc.
negotiate and this always come down - a lot....or just decide that you always wanted to be trussed up and baked on gas mark 3 (and then complain at your weakness later).
(there is NO reason why these couldn't be addressed upfront as the ESS application takes your reg/VIN and the mileage you tell them at the time of booking and adds these items on)
b) extra work that "you might as well" get done while the car is in to avoid possible inconvenience later before the next scheduled service (pads, discs, wiper blades, cabin air filter, aircon "service")
nothing illegal here as you are being offered a convenience.
Learn how to look at the disc pads through the alloys and what they look like worn out and new. Make your own decision on how long they might last - they have wear sensors!
Decide to buy the blades/aircon filter from the parts desk with your discount and fit them in the time it takes to drink a cup or two of coffee.
If the a/c stinks, get a can of Comma stuff from Halfords for £11 and make the carpets and seats smell nice at the same time by running the car on recirc for 10 mins.
you need to make them understand you are savvy customer - not a "Merc experience customer" and that you want your car maintained not spoiled by buying it too many expensive presents.