Guarda el justificante del pago, no sea que te lo pidan en la ventanilla.

Guarda here is the affirmative tú command of guardar: Guarda... = Keep... / Save...

Spanish affirmative tú commands usually look like the él/ella/usted form of the present indicative:

• guardar → guarda
• comer → come
• abrir → abre

So:

• Guarda el justificante = Keep the receipt / proof of payment

You normally do not use the subject pronoun tú unless you want emphasis:

• Tú guarda el justificante = more emphatic, like You make sure to keep it

Justificante is a very common Spain Spanish word for a supporting document that proves something, especially in administrative or formal contexts.

In this sentence, el justificante del pago means something like:

• proof of payment
• payment receipt
• document showing that you paid

It is a bit broader and more official-sounding than just receipt. In many situations, it might be a bank slip, stamped form, confirmation printout, or other document showing the payment was made.

Del is just de + el:

• de el pago → del pago

So el justificante del pago literally means the proof/receipt of the payment.

Spanish often uses this structure where English might prefer a compound noun:

• justificante del pago = payment receipt / proof of payment
• fecha del examen = exam date
• número del pedido = order number

So this is completely natural Spanish.

No sea que is an idiomatic expression meaning:

• in case
• lest
• for fear that

In this sentence:

• Guarda el justificante del pago, no sea que te lo pidan...
• Keep the proof of payment, in case they ask you for it...

Literally, no sea que comes from ser and means something like let it not be that..., but you should usually learn it as a fixed expression.

It is used when someone is being cautious about a possible future situation.

Yes. Sea is the present subjunctive of ser.

Here it appears in the fixed expression no sea que. Even though ser normally means to be, in this phrase you should not translate it word-for-word. Just treat no sea que as a set phrase meaning in case / lest.

So:

• no sea que llueva = in case it rains
• llévate el pasaporte, no sea que te lo pidan = take your passport, in case they ask you for it

Because no sea que triggers the subjunctive.

After expressions involving caution, fear, prevention, or possibility of an unwanted event, Spanish often uses the subjunctive. Here the speaker is warning about a possible future situation:

• no sea que te lo pidan = in case they ask you for it

So pidan is the present subjunctive of pedir.

Compare:

• Te lo piden = They ask you for it / They do ask you for it
  → statement of fact
• No sea que te lo pidan = In case they ask you for it
  → possible situation, not presented as a fact

This is a very useful pronoun combination to break down:

• te = from you / to you
• lo = it

The verb pedir can work like to ask someone for something:

• pedirte algo = to ask you for something

So:

• te lo pidan
• literally: they ask it from you
• natural English: they ask you for it

Here:

• te refers to you
• lo refers to el justificante del pago

Since justificante is masculine singular, the pronoun is lo.

It is an impersonal/unspecified third-person plural.

Spanish often uses ellos/ellas forms without naming the subject when the meaning is people, someone, the staff, officials, etc.

So here:

• te lo pidan en la ventanilla = they ask you for it at the counter/window

The speaker probably means the clerk, office staff, or whoever is attending you there.

This is similar to English using vague they:

• They might ask for ID
• They told me to wait

In Spain, la ventanilla often means the service window or counter in an office, bank, government building, ticket office, etc.

So it does not literally mean an ordinary little window in a wall in this context. It refers to the place where you go up and deal with staff.

Possible translations:

• at the counter
• at the service window
• at the desk
  depending on context

This is a very common word in bureaucratic or administrative Spanish.

Because en la ventanilla here means at the counter/window, that is, in that location or situation.

• en la ventanilla = at the counter
• a la ventanilla would more likely suggest to the counter/window as direction of movement

Compare:

• Ve a la ventanilla 3 = Go to window 3
• Te atenderán en la ventanilla 3 = They will help you at window 3

In your sentence, the idea is where they might ask for the document, so en is the natural choice.

Yes, a similar idea could be expressed with por si acaso, but the nuance is a little different.

• Guarda el justificante del pago, por si acaso. = Keep the proof of payment, just in case.

• Guarda el justificante del pago, no sea que te lo pidan en la ventanilla. = Keep the proof of payment, in case they ask you for it at the counter.

With no sea que, the speaker explicitly mentions the possible problem. It sounds a bit more specific and cautionary.

With por si acaso, the warning is more general unless you add more words:

• Guarda el justificante del pago por si acaso te lo piden en la ventanilla.

That is also correct.

Yes. Pedir means to ask for, to request, or to demand/request something.

That is exactly why it is used here. The staff might ask you for the proof of payment.

Examples:

• Me pidieron el DNI = They asked me for my ID
• Te van a pedir el justificante = They’re going to ask you for the receipt/proof

By contrast, preguntar means to ask a question:

• Me preguntaron mi nombre = They asked me my name
• Me preguntaron dónde vivo = They asked where I live

So in your sentence, pedir is the right verb because it refers to requesting a document.

It is still perfectly normal and common, especially in spoken and written Spanish when giving a warning or precaution.

It can sound slightly more careful or idiomatic than a plain por si acaso, but it is not unnatural or literary in the wrong way.

Examples you might really hear:

• Llévate una chaqueta, no sea que refresque.
• Guarda la factura, no sea que luego la necesites.
• Sal ya, no sea que llegues tarde.

So it is a useful expression to learn.

Sometimes yes, depending on context, but keep is usually safer here.

• Guarda el justificante = Keep the proof of payment
• In some contexts: Save the receipt

In English, save can sound more natural for digital things or for setting something aside for later. Keep works very well for physical documents and is the most straightforward translation here.

Because Spanish very often uses object pronouns once the object is already clear.

Instead of repeating:

• no sea que te pidan el justificante en la ventanilla

Spanish often prefers:

• no sea que te lo pidan en la ventanilla

Both are possible, but the version with lo sounds very natural because el justificante has just been mentioned.

This is common in Spanish:

• Trae el pasaporte. No lo olvides.
• Lleva la factura, por si te la piden.

Yes, depending on the exact situation.

Possible alternatives include:

• recibo = receipt
• resguardo = stub / receipt / proof slip
• comprobante = proof / voucher / receipt
  though comprobante is more common in many Latin American varieties than in Spain

In Spain, justificante is especially common in administrative and formal situations, which is why it fits this sentence very well.

Yes, very much so.

A few things make it sound very natural in Spain:

• justificante in an administrative sense
• ventanilla for a service window/counter
• the use of no sea que
• te lo pidan with an unspecified they

So this sounds like something you might really hear when dealing with bureaucracy, a bank, public administration, university paperwork, and similar situations.