When someone uses letters to indicate items, either uppercase or lowercase letters are correct; however, uppercase letters are probably more readable and less likely to get lost in the flow of the sentence.
...We needed, A, money and, B, time.
...I knew that, A, he was lost and, B, that he would need time to get back on the right path.
Note: The letters are always surrounded by punctuation.
Happy punctuating!
Margie