Carteiro vs. carteira
While the Portuguese words carteiro and carteira look very similar and are both related to the word carta (letter), they refer to completely different things. The primary distinction is that one refers to a person and the other almost always refers to an object.
Carteiro
A2This is a masculine noun (o carteiro) that refers to a person whose job is to deliver mail. The English equivalents are mailman, postman, or mail carrier. The feminine form for a female mail carrier is a carteira, which is identical to the word for wallet.
O carteiro entregou uma encomenda hoje de manhã.
(The mailman delivered a package this morning.)
Meu cachorro sempre late quando o carteiro se aproxima.
(My dog always barks when the postman approaches.)
Perguntei ao carteiro se ele tinha alguma correspondência para mim.
(I asked the mail carrier if he had any mail for me.)
Carteira
A1This is a feminine noun (a carteira) that most commonly refers to an object used to hold money, cards, and documents, i.e., a wallet or purse. By extension, it also refers to various types of official cards, such as a driver's license or a professional ID.
Perdi a minha carteira com todos os meus documentos e cartões.
(I lost my wallet with all my documents and cards.)
O policial pediu a minha carteira de motorista.
(The police officer asked for my driver's license.)
Ela guarda uma foto da família dentro da carteira.
(She keeps a family photo inside her wallet.)
Para exercer a profissão, ele precisa da carteira do conselho regional.
(To practice the profession, he needs the ID card from the regional council.)
Você precisa apresentar sua carteira de trabalho na entrevista.
(You need to present your work card at the interview.)
A nova carteira que entrega as cartas no bairro é muito simpática.
(The new mailwoman who delivers letters in the neighborhood is very nice.)
Summary
In short, o carteiro is a masculine noun for a person: the mailman. A carteira is a feminine noun that almost always refers to an object: a wallet, a purse, a driver's license, or an ID card. Although a carteira can also mean a female mail carrier, this usage is far less common and the context usually makes the meaning clear. The simplest way to remember is to associate carteiro (ending in -o) with the male person and carteira (ending in -a) with the object.