1. What is the opposite of "upside down"?
2. Don't tell me, because I already bought many postcards for you.
2-1. Don't tell me, for I already bought many postcards for you.
2-2. Don't tell me because I already bought many postcards for you.
(What is the difference among the three? Do we have to put a comma before 'for' or 'because'?)
The opposite of "upside down" is "rightside up".
2. "because I bought many ....." is an adverb clause modifying "do tell".
Clauses need to be as near the word they are modifying as possible in order to avoid confusion.
Since the clause is at the end of the sentence, it needs to have a comma between it and the main sentence.
The word "for" is not usually used in that context. So the best sentence is #1.
2