In the case of the preposition pelo(a) — Por + a/o, it needs to match the noun's gender, but most of the times we wouldn't even use the article.
Vou lá na quitanda comprar uva, banana, maracujá, alface, cenoura etc.
Traz uva para a gente, por favor.
Nunca vi uma pessoa que gosta tanto de uva como ela.
Quantos cachos de uva é para eu comprar? — informal sentence.
Quantos cachos de uva você quer que eu compre? — A bit more formal, but OK.
My answer may not be so accurate on regions other than the São Paulo State or even the Metropolitan Region of Vale do Paraíba e Litoral Norte.
I have seen in other sentences in this lesson that "eu vou......" "nós vamos......." can be translated "I will.... OR I am going to......" "we will...... OR we are going to....." Is this always the case when the verb "ir" is used to show future tense? Are there always the two options?
This is odd. Portuguese seems to be a language where the article is used where English does not need it (e.g. O meu pai), yet here the article is omitted in Portuguese but required in English. Admittedly, "I will pay for grape" is a stupid looking translation but on this one I found myself second-guessing Duolingo and getting it wrong.
"pagar" means pay/to pay, not pay for. "pela" combines "for" (por) and the definite article "the" (a). So, "pagar pela" = "pay for the".