It is business and it wouldn't be very successful if they didn't try to get ever little bit. Them asking you if they can do anything else is no different than the tire ads hanging on the wall or special signs on the wall. The only difference is it's verbal. When you walk into the shop and read the ads you say to yourself do I need this or want this and then you tell yourself yes or no. Same difference, it's just a sign that is speaking to you. You either need something else or you don't. Whether you say your answer out loud or not is up to you but remember when they ask you all they are doing is their job.
By asking the question it can help build trust. If you went into a parts house with a buddy to get brake pads for your truck and your buddy asked you (like all friends would) "do you need any brake fluid or brake cleaner?" Would you think he was trying to make you spend money? No you would think he is looking out for his bud and making sure you didn't have to make a special trip back for something.
There are little tricks to the trade that are used to up sale like cheap oil changes to get your hood open or free rotations to get your wheel off. This does a couple things. It allows them to check points that they could not without getting to those spots to help make your vehicle more reliable and/or safe and also get an up sale. For people like me that do there own maintenance and repairs this can get annoying at time but remember they are the little guy just doing their job. Believe me when I say that when you work in one of these shops and you deal with the customers that come in day in and day out that do well to know which side the gas cap is on more less check fluids under a hood that they don't know how to open you have to cover all bases. Until they get to personally know you you will be treated like the customer that can't find the hood latch.
I'll never for get the time I had a customer come in on a roll back with a no start no crank condition. You do the normal checks, battery/starter/fuses/wires/etc. Then you call them and start asking questions.Come to find out she decide that she was going to check the oil in her car. Hey, she got the hood up! She then proceeded to unscrew the oil cap and look under it for oil. Yep! looked under the cap that said oil. Needless to say, I don't remember how much oil it took but when I went and pulled the oil filler cap it was full to the top. I felt really sorry for the lady because she was actually trying to do right and ended up pouring a lot of money down that little opening 1 quart at a time. This is a true story BTW.