Metto i surgelati alla fine della spesa, così restano freddi più a lungo.

Metto is the first-person singular form of mettere, meaning I put or I place.

Italian often uses the present tense to talk about habits or routine actions, just like English does. So here it means something like:

I put the frozen foods in last when I shop.

It is not necessarily describing only what the speaker is doing right now.

I surgelati means the frozen foods or the frozen products.

Here, surgelati is originally an adjective, but it is being used like a noun. This is very common in Italian. Literally, it means the frozen ones, but in a shopping context it naturally means frozen items/foods.

• il surgelato = a frozen product
• i surgelati = frozen products / frozen foods

So i surgelati is a normal supermarket expression.

Both relate to something being frozen, but they are not always used in exactly the same way.

• surgelato usually refers to commercially frozen food, especially food sold in the freezer section
• congelato is more general and can mean frozen in a broader sense

In a grocery-shopping sentence like this, i surgelati is the most natural choice for frozen foods.

Literally, it means at the end of the shopping.

Breakdown:

• alla fine = at the end
• della spesa = of the shopping / of the grocery shopping

In natural English, the whole phrase means something like:

at the end of my shopping or last, when I do the shopping

The idea is that the speaker picks up the frozen items near the end, so they do not warm up while the rest of the shopping is being done.

In Italian, la spesa very often means grocery shopping or the groceries.

A very common expression is:

fare la spesa = to do the shopping / to buy groceries

So in this sentence, la spesa refers to the shopping trip itself, not to money being spent.

They are combinations of a preposition + definite article.

• a + la = alla
• di + la = della

So:

• alla fine = at the end
• della spesa = of the shopping

These combinations are extremely common in Italian and are something learners see all the time.

Here così means so, this way, or so that.

It introduces the result of the action:

Metto i surgelati alla fine della spesa, così restano freddi più a lungo.

= I put the frozen foods in last, so they stay cold longer.

So così links the first part and the consequence of it.

Restano is from restare, meaning to stay or to remain.

Here it means:

they stay cold longer

The subject is i surgelati, which is plural, so the verb is plural too:

• resta = it stays
• restano = they stay

Yes, rimangono from rimanere would also be possible in many contexts. Both restano freddi and rimangono freddi can mean they stay cold. In this sentence, restano sounds completely natural.

Because freddi has to agree with i surgelati.

• i surgelati = masculine plural
• so the adjective must also be masculine plural
• freddo = masculine singular
• freddi = masculine plural

This is basic adjective agreement in Italian.

Più a lungo means for longer or longer in the sense of for more time.

• a lungo = for a long time
• più a lungo = for a longer time

This is the natural way to talk about duration here.

By contrast, più lungo usually refers to physical length:

• un vestito più lungo = a longer dress
• una strada più lunga = a longer road

So in this sentence, più a lungo is correct because we are talking about time, not size or length.

Yes, the subject pronouns are omitted, and that is very normal in Italian.

• metto already tells you I put
• restano already tells you they stay

Italian often leaves out subject pronouns when the verb ending makes the subject clear.

So:

• Metto = I put
• Restano = they stay

You could add io or loro, but usually only for emphasis or contrast.

Usually, così is better here.

Così expresses the result of the action:

I put the frozen foods in last, so they stay cold longer.

If you use perché, it sounds more like you are giving a reason:

I put the frozen foods in last because they stay cold longer.

That is understandable, but logically the second part is really the consequence of putting them in last. So così fits the sentence more naturally.

If you wanted a reason structure, a more natural alternative might be:

Metto i surgelati alla fine della spesa per farli restare freddi più a lungo.

= I put the frozen foods in last to keep them cold longer.