Haneda Airport Post Office?

Hey everyone, i'm going to arrive in Japan from Indonesia at the 3rd of March (Tuesday) around 9AM or so. I'm pre-booking Mobile Wifi in which i'd like it to be delivered to the airport for convenience. I managed to do a bit of googling, and got several questions i've yet to find the answer;

1. Is the post office at Haneda airport only located in the DOMESTIC terminal?

2. If so, what is the best option for me to reach the Domestic terminal?

3. If by rail, is it subject to fare charges?

4. I'm going to be traveling to Osaka by Shinkansen after i retrieved my Wifi and Rail pass. If i were to reach the station that serves Shinkansen, will it be better for me to board the train at SHINAGAWA station? or should i still proceed to board the train at TOKYO?

Appreciate the help people, thanks in advance!

1. Is the post office at Haneda airport only located in the DOMESTIC terminal?

Yes, at terminal 1.

2. If so, what is the best option for me to reach the Domestic terminal?

There are free shuttle buses between the terminals.

3. If by rail, is it subject to fare charges?

Rail and monorail would cost a small fee.

I'm going to be traveling to Osaka by Shinkansen after i retrieved my Wifi and Rail pass. If i were to reach the station that serves Shinkansen, will it be better for me to board the train at SHINAGAWA station? or should i still proceed to board the train at TOKYO?

From Haneda it is much faster and easier to reach Shinagawa. The only reason why Tokyo could be of advantage is that you intend to travel by unreserved seat during a busy period. Then it would be better to board at Tokyo Station because it is the first station and comes with better chances to get an unreserved seat.

For penny pinchers, the trip to Tokyo is also covered by the JR Pass, whereas the direct trains to Shinagawa are not.

wow, thanks for the response everyone! i got it now, i think i'm opting for the Tokyo station instead. Cheers!