Narita Term. 2 Postal Service

Can anyone confirm the times the postal service in terminal 2 at Narita Airport? I arrive at 4 PM on Saturday and the website says it's open until 8 PM 365 days a year so that should include Saturdays obviously. However, the company that is picking me up says that's not true and it closes at 5.

You are right.
The website information is correct.
(Most of post offices close at 5pm/6pm in weekdays.)

But please note that it seems there's no ATM machine there and they offer postal service only.

There is a JP Bank ATM at Narita, though, but it's located with the other ATMs, not next to the post office.

i really don't think the OP has any intention of needing to use the atm's - as there are plenty of money options at narita, he asked about the post office so more likely is picking up mail.

The post offices at narita are nothing more than service counters and they do open till 20:00 every day.