Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.
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Question: I was writing something along '... I could only read upto page 34' when my spell-check app Grammarly complained that I should rather use 'up to'. So, what's the difference between the two and is there really any difference? Or is it that the condensation 'upto' is not technically allowed?.

Answer: Upto is a misspelling of up to. -- Wiktionary Use up to with a space between the two words. Also, as Yosef Baskin says: You can use the following self-test: Does the single word say something different from the two words? If not, stay with two words. By contrast, into and onto do mean things other than in to and on to, so they work as single words.

Question: The context: Customer pays for some service 100$. If he chooses an option X, we charge him 20$ more, so 120$ in total. As a programmer I want my variables to have meaningful names. How would you call this 20$ in this context? I consider two words: markup (but it is more related to the business than customer IMHO) and surcharge.

Answer: The words that immediately come to mind are 'surcharge' and 'fee'. Consider the following: Basic cable service provides 50 standard channels for a set price of $30 a month. As for the premium channels, you may order them for an additional surcharge of $20. ...or the following: Standard repair service rate for a computer is $40 an hour. There is, however, an overnight fee of $10 for every night the computer is kept in our possession, after the repair is completed. We notify you after we have completed the service, and for every day that passes, you incur this extra fee. I am just utilizing random numbers on the example sentences, but I hope they help clarify usage.

Question: How does one correctly apply 'in which', 'of which', 'at which', 'to which', etc.? I'm confused with which one to apply when constructing sentences around these.

Answer:
The trick to knowing how to use; of which, at which, in which, to which, from which is to analyse the prepositional phrases, phrasal verbs, verbs and prepositions: He /spoke of/ war and peace and many other topics that day. The topic /of which he spoke/ was complex. The verb here that means to speak about a topic is /to speak/ of a topic/: to mention The party /at which/ he spoke/ was noisy. Phrase: A party is held /at a place/. It is implied. The situation /in which/ we found ourselves was dire. Phrase: /to find oneself /in a situation. The bonds /from which/ we broke free were tight. phrase: to /break free/ from bonds. The town /to which/we were driving was 50 ks away. phrase: to /drive/ to a place. Summary (and not a complete answer but a general one): The preposition depends on the verb that takes a preposition, a phrasal verb that includes a preposition, or it depends on the prepositional phrase used. Also, there are many other prepositions that can be paired with which: under, during, about, over, etc.