"The last letter is mine."
Translation:A última carta é minha.
Almost all adjectives can be placed before or after the noun, but they sometimes have different meanings depending on their placement. I'm not sure if "carta última" is grammatically incorrect here, but you'll never hear any native speaker putting the adjective "última" after the noun. I guess it's just an exception to this rule, and that goes to other adjectives related to 'position' as well, like primeiro (first), terceiro (third), etc. You always use those before the noun.
Actually, you can say "o carro primeiro", but it doesn't mean "the first car", it means "the car first", like in "the car comes first".