Sao Paulo For Lunch
As Itaunas said, these are everywhere, and there will be multiple options wherever you are to choose from. I had a great lunch in Sampa at Dona Deola in Vila Madalena, on Pompeia.
There's a place in my heart for Lanchopperia Bandeirantes on Augusta at Alameda Jau in Jardins. Great salgados, nice lanches, and strong caipirinhas.
For this kind of eating I think most hounds just wander around and choose a place rather than working off a list. And you can do it just about anywhere: center districts, shopping malls, but the commercial centers of the capital cities have the best options (in the interior where feasible a lot of people lunch at home). Rio Centro is the best overall area for lunch plates and per-lb buffets, there is a lot of variety. But a number of hounds have posted other specific recs in the past (and from what I recall streetgourmetla has other SP recs in this vein on his blog).
I was glad to see this article because a lot of US eaters seem to have a stigma about per-lb restaurants ("looking for a Brazilian rec, no per-lb"). There are plenty of excellent per-lb Brazilian restaurants in Brazil, plus there are buffets where you can try a large number of plates from a certain region such as a restaurant offering food from Minas Gerais. It also introduces folks to the ubiquitous PF (prato feito) via its dandied up name "prato commercial" (prato executivo is also used) -- it all means meat, rice, beans, farofa and likely a salad. Plus when you order off that part of the menu at an a la carte restaurant the ample serving is generally intended for one as opposed to an a la carte plate which serve two. At the lower end many per-lb places also have a PF: you get a limited choice of protein (2 meats is common) and they make the plate for you, a marmitex (or quentao) is the to go version of the PF. (Since a PF or marmitex will come with a lot of rice, etc, one way for a couple to lunch is to get one PF and then get a separate per-lb portions of whatever meats you want to try.)
What I think the article should also mention is ordering a "porcao" (a serving or side plate) at bars and some restaurants. If you are travelling its sometimes nice to order a pastel and suco just before 11am and then around 2-3pm sit at a bar for a porcao like "frango a passarinho" or "carne de sol," "torresmo pururuca" (pork rinds) or "linguica frita" etc. Even with the smaller serving a PF can be very filling and its so easy to "beliscar" in Brazil whether its from pastelarias, feiras, tapioca carts, baianas, petiscarias and then you aren't walking around totally full.
Language-wise in addition to comanda the article should also mention "ficha" (token) since its so common, for instance many coffee or lunch counters will direct you to "retira ficha na caixa" or pay for what you plan to order first and present the ticket to the server. Although many let you do the reverse, doing it in the right order will save you a lot of time especially in a busy city center.