Guarda el justificante junto con la documentación, por si te lo piden en la ventanilla.

Yes. Guarda is the tú imperative of guardar, so it means keep or save as a direct instruction.

• Guardar = to keep, save, put away
• Guarda = keep/save! (said to one person informally)

If the speaker were being formal, it would be guarde.
If they were speaking to more than one person in Spain, it would be guardad.

In Spain, justificante usually means a document that proves something, such as:

• proof of payment
• a receipt
• proof that you submitted something
• a confirmation slip

It is broader than just receipt. The key idea is supporting proof.

For example, a bank transfer receipt, an appointment confirmation, or proof that you paid a fee could all be called a justificante.

They can overlap, but they are not exactly the same.

• recibo usually means a receipt or bill, especially for a payment
• justificante is any document that serves as proof

So a recibo can be a justificante, but justificante is the wider term.

In bureaucratic or administrative Spanish, justificante is very common because offices often want proof, not just a simple till receipt.

Both are possible, but junto con emphasizes the idea of together with or along with.

• con la documentación = with the documentation
• junto con la documentación = together with the documentation

So junto con sounds a bit more explicit: keep that proof together with the rest of the paperwork.

Documentación is a collective noun. It refers to the set of documents or paperwork as a whole.

So:

• la documentación = the documentation / the paperwork
• los documentos = the individual documents

Spanish often uses documentación in administrative contexts where English might say paperwork.

Por si means in case.

It introduces a precaution: do this now because something might happen later.

So the structure is:

• Guarda el justificante... por si...
  Keep the proof... in case...

This is a very common Spanish expression for practical advice and warnings.

Because por si is normally followed by the indicative, not the subjunctive.

That is one of the most important things to remember about this expression.

So:

• por si te lo piden = in case they ask you for it

Even though the action is possible or future, Spanish still uses the indicative after por si.

English speakers often expect a subjunctive here, but standard Spanish says:

• Llévalo por si lo necesitas
• Te llamo por si pasa algo
• Guárdalo por si te lo piden

These are object pronouns.

• te = from you / to you, depending on context; here it means they ask you for something
• lo = el justificante

So te lo piden means literally something like:

• they ask you for it

In smoother English: they ask you to show it or they ask you for it.

Piden is they ask/request.

In this sentence, the subject is not stated because Spanish often leaves it implied. Here it refers to some unspecified people, such as:

• the office staff
• the clerk
• the administration
• the people at the counter

Spanish often uses the third person plural in this vague way when the exact people are not important.

Literally, ventanilla means little window, but in Spain it very often means a service window, counter, or clerk’s desk in places like:

• government offices
• banks
• stations
• administrative buildings

So here it refers to the place where you deal with a clerk, not an ordinary house window.

It is a very common word in bureaucratic Spanish.

Using la makes it sound like a known or expected place in that context: the service window you will go to as part of the procedure.

Spanish often uses the definite article in these practical, situational expressions where English might be more flexible.

So en la ventanilla sounds natural if both speaker and listener understand the kind of office process being discussed.

It is very natural here because por si te lo piden en la ventanilla adds an explanation of purpose or precaution.

The comma helps separate:

• the main instruction: Guarda el justificante junto con la documentación
• the reason/precaution: por si te lo piden en la ventanilla

You may sometimes see similar sentences without a comma, but with a longer sentence like this, the comma is very helpful and standard.

Yes. The most obvious formal change would be the imperative:

• informal: Guarda el justificante...
• formal: Guarde el justificante...

You could also make the whole sentence sound more formal depending on the situation, but that single change is the main one.

In Spain, whether tú or usted is used depends on the relationship and level of formality.