Se a sacola ficar muito cheia, eu deixo tudo no carrinho até chegar ao caixa.

Here, ficar is actually the future subjunctive, not just the infinitive.

After se when Portuguese talks about a possible future situation, it often uses the future subjunctive:

• Se a sacola ficar muito cheia... = if the bag gets too full...
• Se ele chegar cedo... = if he arrives early...

With many regular -ar verbs, the future subjunctive looks exactly like the infinitive, so ficar can look confusing.

Compare with an irregular verb:

• se eu for = if I go
• se eu tiver = if I have

So in your sentence, ficar is the correct form for a future possibility.

Ficar often means to become / to get, while estar means to be.

So:

• ficar muito cheia = to get too full
• estar muito cheia = to be too full

In this sentence, the idea is that the bag reaches that state as you keep putting things into it. That is why ficar sounds natural.

If you said Se a sacola estiver muito cheia, that would mean If the bag is too full. That is also possible, but it focuses more on the condition itself, not the process of getting full.

Because cheia agrees with a sacola, which is a feminine singular noun.

Agreement in Portuguese:

• o saco está cheio
• a sacola está cheia
• as sacolas estão cheias

So cheia matches sacola.

Portuguese often uses the present indicative to talk about:

1. habitual actions
2. planned/expected future actions, depending on context

Here, eu deixo tudo no carrinho can sound like a habit or a normal procedure:

• If the bag gets too full, I leave everything in the cart...

It is similar to English using the present in a general rule:

• If it rains, I stay home.

You could also say:

• eu vou deixar tudo no carrinho
• deixarei tudo no carrinho

But eu deixo sounds very natural in everyday speech.

Sacola usually means a shopping bag, especially the kind you carry groceries or store items in.

In Brazil, it often refers to:

• a plastic store bag
• a reusable shopping bag
• a grocery bag in general

It is more specific than just any kind of bag. For example:

• bolsa usually means a purse/handbag
• mala means suitcase
• saco means sack/bag, but not the usual word here

So sacola is a very natural word in a shopping context.

Here carrinho means a shopping cart or shopping trolley.

It comes from carro or carrinho, literally something like little cart. The -inho ending is a diminutive, but in many cases it is just the normal word people use.

In a store context:

• carrinho = shopping cart
• cesta = basket

So this sentence is talking about leaving the items in the cart instead of putting them into the bag right away.

In this context, caixa usually refers to the checkout area, cash register, or by extension the cashier.

In Brazilian Portuguese, caixa can mean:

• the register itself
• the checkout line/spot
• the cashier person, depending on context

So chegar ao caixa means to get to the checkout.

You may also hear:

• fila do caixa = checkout line
• ir para o caixa = go to the checkout

Because the verb chegar normally takes the preposition a:

• chegar a algum lugar = to arrive at some place

So:

• chegar ao caixa = chegar a + o caixa

That contracts to ao.

This is the same pattern as:

• chegar ao mercado
• chegar à loja
• chegar ao trabalho

Using no caixa would not fit as well with chegar in standard Portuguese. No caixa is more like at the checkout, not arrive at the checkout.

Because Portuguese often omits the subject when it is already clear from context.

In eu deixo tudo no carrinho até chegar ao caixa, the understood subject of chegar is the same eu from the main clause.

So it means:

• I leave everything in the cart until I get to the checkout.

You could make it more explicit and say:

• até eu chegar ao caixa

That is also correct, but not necessary here.

Tudo means everything or all of it.

Here it refers to all the items being bought. In natural English, that would often be:

• everything
• all the items
• all my things

Portuguese uses tudo very naturally in this kind of situation. You could also say something more specific, like:

• deixo as compras no carrinho = I leave the groceries/purchases in the cart

But tudo is simple and very common.

Yes, that would also be grammatical, but the nuance changes slightly.

• Se a sacola ficar muito cheia... = if the bag gets too full
• Se a sacola estiver muito cheia... = if the bag is too full

So:

• ficar emphasizes the bag becoming full
• estar emphasizes the bag already being in that state

Both are natural; the original sentence just highlights the idea of the bag filling up.

It can easily be understood as a general habit or usual procedure:

• If the bag gets too full, I leave everything in the cart until I reach the checkout.

That is why the present tense sounds good.

Depending on context, it could also refer to a future situation, but without more context, it most naturally sounds like something the speaker usually does.

Yes, in some contexts you could shorten it, but the meaning is a little less explicit.

• até chegar ao caixa = until reaching/getting to the checkout
• até o caixa = until the checkout / up to the checkout

The original version is clearer because it includes the action chegar. It directly expresses until I get to the checkout.

So até chegar ao caixa is a very natural full version.