Iphone X

There are no iPhone retailers at airports in the US. Given the shortage of iPhone X, I don't think you are going to be able to order one and get it anytime soon. Mail orders are 3-5 weeks, and pickup at an Apple store cannot even be scheduled if a store does not have any devices.

You're welcome. If you have a friend in the US as the other person stated, be advised at this point, Apple has not released a SIM-free version. Also, you have to be concerned about which version of the phone will work with the LTE bands in your country, and the warranty is not international. If there is a problem with your phone, you would have to bring it back to the US for warranty service.

Chris: I think to travel to USA and buy an Iphone X, and use when I return to Argentina

Should I have to pay anything else the full price to use in Argentina?

Which operator I have to choise? Verizon could be?

Regards

Pickup in the US is currently available only at Apple stores.

And possibly some retailers - check their websites.

AFAIK, there is no pickup available at airports in the US.

I would suggest you to ask for help from a friend who's staying in the US.