Thanks for the explanation below, that's good to know. But when I hover over the word it says 'box' in the list of translations and says it is masculine. So 'o caixa' being taught by duolingo also means box - should it be corrected by someone?
Gosh, when I saw "o caixa" for a minute I thought I had always used the wrong article telling my BR customers that the boxes of their spare part orders were ready...! XD Luckily I just learnt something new & interesting, thank you Duo & thank you Paulenrique!