Como dizer “Só isto” em inglês

Nobody ever says "just it" in that context, it just doesn't sound like very good English. Out of "just it" and "just that" the latter is a much more advisable and better option to turn to when faced with such context in a conversation but "that's all" is the one that the vast majority of people would use.

To be more thorough on that matter and make sure nothing is left up in the air, It seems to me like "just it" would only be used in a context where the "just" would come in as a sort of emphasizer, like when you say "that's it" and then turn it into "that's just it" meaning that something has come to an end and there's nothing else left to be said or done, and the "just" is working more as a "simply" rather than a quantifier that's indicating there's nothing else you want to add (like in the context you presented us with where you're grocery shopping and the cashier asks if that's all you're buying).

Vlw PPAULO! However, I think we had a miss understanding, actually, I’d like to know how to answer that I’m okay!

Could I say “just that”? Or I must say, “just it!” Or these sentences are interchangeable?

Thanks!

PS: if you don’t know the answer, whatever, I really appreciate your answer above!

Thanks in advance!

Could be: is that all? - in Portuguese "isso é tudo?/é só isso?"
Is that all I get? Or is that all there is? Also does the trick, but it´s less concise, so the complete version of it.
Somehow in the hope that the listener/giver, etc, will say "no, there is more".

I am not going to be definite on this one. But my educated guess is that, yes he could ask "is that all?" and you would answer "yes, that´s all" (all you want is a box of chocolate, you don´t want anything more).
But he could ask "just one (box)?" - yes, just one (not a second box). Or he could say "anything more?" not "that´s all" (not a second box or anything else - not other items).

I tried to imagine a variety of contexts, and even so there is the likelihood that I am not 100% accurate, so let´s wait for the advanced and native speakers (of English) to confirm, correct or add some details and new insights.
Mine are educated guesses, I would like additional opinions.
This is an interesting question indeed.

Makes a whale of a sense, Leonardo. Thanks for mentioning that (important) detail as well.