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?
Which suggests that originally it was a type of box, a cash box with a means of registering the cash paid into it. And over time it's just been shortened to caixa, which doesn't make sense without the history, but I'm sure there are lots of things like that in English.
I wrote "Where is the till?" and it was rejected suggesting me to use "a teller". What the hell is a teller and why is "till" deemed wrong? Everyone says so in the UK and Ireland! I can't even report for some reason...
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!