No te olvides de guardar el justificante del pago en la cartera.

Because this is a negative command: Don’t forget.

In Spanish, negative commands for tú use the present subjunctive, not the normal present tense.

• te olvidas = you forget / you are forgetting
• no te olvides = don’t forget

So here:

• No te olvides... = Don’t forget...

A useful comparison:

• Olvídate = Forget... / Don’t worry about it (affirmative command)
• No te olvides = Don’t forget

Te is the reflexive pronoun for tú.

The verb here is olvidarse de, which is very commonly used to mean to forget.

So:

• olvidar = to forget
• olvidarse de = to forget

Examples:

• Olvido las llaves. = I forget the keys.
• Me olvido de las llaves. = I forget the keys.

In everyday Spanish, olvidarse de is extremely common, especially when followed by a noun or an infinitive.

Because the structure is:

• olvidarse de + infinitive

So if you want to say to forget to do something, Spanish often uses:

• olvidarse de + verb in the infinitive

Examples:

• Me olvidé de llamar. = I forgot to call.
• No te olvides de cerrar la puerta. = Don’t forget to close the door.
• No te olvides de guardar... = Don’t forget to put away / keep...

So the de is required by this verb pattern.

Yes. That is also correct.

There are two common ways to express this idea:

• No olvides guardar...
• No te olvides de guardar...

Both mean Don’t forget to keep/put away...

The version with olvidarse de can sound a little more natural or conversational in many contexts, but both are standard.

Very roughly:

• No olvides... = a bit more direct
• No te olvides de... = very common, slightly more idiomatic in everyday speech

Because in negative commands, object and reflexive pronouns go before the verb.

So:

• No te olvides. = correct
• No olvides-te. = incorrect

Compare:

• Olvídate. = affirmative command, pronoun attached to the end
• No te olvides. = negative command, pronoun before the verb

This is a very important pattern in Spanish:

• Siéntate. / No te sientes.
• Lávate las manos. / No te laves las manos.

Justificante is a document that serves as proof of something.

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

• the payment receipt
• the proof of payment
• the payment confirmation

It is a little broader than English receipt, because it focuses on the idea of evidence or documentation showing that the payment was made.

In real life, it could be:

• a printed receipt
• a bank slip
• a payment confirmation document
• proof from an app or transfer

Del is the contraction of:

• de + el = del

So:

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

This is completely standard Spanish.

Other examples:

• el nombre del hotel = the name of the hotel
• la puerta del coche = the car door
• el final del libro = the end of the book

Note that this contraction happens with de + el, but not with de + la:

• de la cartera stays de la cartera

Here guardar means something like:

• to keep
• to put away
• to store
• to save (in the general sense of keeping something safe)

In this sentence, the most natural English idea is probably:

• keep
• put away

So guardar el justificante en la cartera means:

• to keep the receipt in your wallet
• to put the proof of payment in your wallet

It does not mean save in the computer-file sense here.

In Spain, cartera very often means wallet.

So in this sentence, en la cartera is most naturally:

• in your wallet

Depending on context, cartera can also mean:

• purse
• briefcase
• schoolbag (less likely here)

But because the sentence talks about keeping a payment receipt, wallet is the most likely meaning.

This is worth noting because in some varieties of Spanish, other words may be more common for wallet, such as billetera or monedero.

Spanish often uses definite articles more naturally than English does.

So:

• el justificante
• del pago
• la cartera

sound normal and natural in Spanish, even where English might prefer:

• the receipt
• proof of payment
• in your wallet

In context, these articles help present the items as specific or understood:

• the payment receipt
• the wallet

Also, Spanish often uses the definite article where English might use a possessive. For example, Spanish may say la cartera where English would naturally say your wallet, if the owner is already clear from context.

No. Here en is the correct preposition because it means in / inside.

• en la cartera = in the wallet

If you used a, it would suggest movement to something, which would not sound right with this verb in this sentence.

Compare:

• Guardar algo en la cartera = to put/keep something in the wallet
• Ir a la cartera would mean something like to go to the wallet, which is a different idea

So en is used because the receipt ends up inside the wallet.