El correo certificado tiene una tarifa postal.

Because correo is a masculine noun in Spanish, so it takes the masculine singular article el.

• el correo = the mail / the post / the postal service
• la correo is incorrect

This is something you simply have to learn with the noun.

In many contexts, correo by itself can mean either mail or email, depending on the situation.

But in correo certificado, it clearly means postal mail, not email. That is because certificado in this expression refers to a postal service such as registered or certified mail. So the phrase points to physical mail.

Here, certificado is an adjective describing correo. It means something like certified or registered in a postal sense.

So correo certificado is a fixed expression for a type of mail service where the item is officially recorded, tracked, or handled with proof of mailing or delivery, depending on the country.

Also note that certificado can be a noun in other contexts, where it means certificate, but that is not what it is doing here.

In Spanish, adjectives often come after the noun, especially when they classify or identify a type of thing.

So:

• correo certificado = certified mail
• tarifa postal = postal rate

This is very normal word order in Spanish. English often puts adjectives before the noun, but Spanish commonly places them after.

Because the subject is el correo certificado, which is grammatically singular.

The main noun is correo, and certificado is just describing it. Since correo is singular, the verb must also be singular:

• El correo certificado tiene... = singular
• Los correos certificados tienen... = plural

So tiene is the correct third-person singular form of tener.

Tiene comes from tener, meaning to have. The sentence is presenting certified mail as something that has a postal rate.

That is why tiene works here.

Compare:

• El correo certificado tiene una tarifa postal. = Certified mail has a postal rate.
• Hay una tarifa postal para el correo certificado. = There is a postal rate for certified mail.
• Es una tarifa postal. = It is a postal rate.

All of these are grammatically possible in different contexts, but they do not say exactly the same thing. The original sentence uses tiene to connect the service directly with the fee.

Because tarifa is a singular countable noun, and in Spanish, singular countable nouns usually need an article or determiner.

So:

• una tarifa postal = a postal rate
• la tarifa postal = the postal rate

Leaving out una would sound incomplete in normal Spanish.

Because postal is an adjective meaning postal or related to the mail/post office. Spanish often uses an adjective after the noun where English might use a noun or another structure.

So:

• tarifa postal = postal rate / postage rate
• servicio postal = postal service

You could sometimes express a similar idea with de, depending on the phrase, but tarifa postal is a very natural standard expression.

Because not all Spanish adjectives have separate masculine and feminine forms.

Postal ends in a consonant, and its singular form is the same for both masculine and feminine:

• servicio postal = masculine singular
• tarifa postal = feminine singular

Only the plural changes:

• servicios postales
• tarifas postales

So postal already agrees correctly with tarifa.

Not naturally. Cuesta means costs, so it needs an amount or price, not a category like tarifa postal.

For example:

• El correo certificado cuesta 10 pesos. = Certified mail costs 10 pesos.

But tiene una tarifa postal means that this service comes with or is subject to a postal rate/fee. That is a different idea.

So in the original sentence, tiene is the better choice.

Yes, correo certificado is widely understood across Latin America. However, exact postal terminology can vary somewhat by country or institution.

In some places, people may also use other expressions depending on the service offered, but correo certificado is a standard and broadly understandable term for this kind of official mail service.