Hi! You can say "As cartas são minhas" or "São minhas cartas"
You ask about the phrase "As minhas cartas" but missing the verb "são". The sentence is not complete, because do not have the verb "to be".
As / cartas / são / minhas The / letters / are / mine
As / minhas / cartas The / my / letters
It is not wrong, but is not complete.
no, the verb SER is the one used to indicate possession , "they are mine", if you use the ESTAR you may want to say "the letters are with me" or even "i have the letters (right now)" but you won't be able to say they belong to you, and you wouldnt phrase it like "está minhas" you would say it as "as cartas estão comigo" (they are here, with my person)
"cartas" is plural and is the subject of the sentence. If you were to substitute a pronoun for "cartas" (letters), that pronoun would be "ellas" (they). You would not say "They is" you would say "They are". São translates to "are" whereas è means "is". My accent might be in the wrong direction. I hope this helps with you understand subject-verb number agreement.