Seat selection/reservation for popular trains

On my next trip to Japan I am planning to take a few popular tourist trains like Resort Shirakami and Saphir Odoriko. Train reservations can be made on Eki-Net 30 days in advance of travel even from overseas but not seat selection.

Is there a way to make seat selection from overseas? Are there any reliable and reasonably priced vendors who could make these reservations with specific seat assignment and deliver the tickets to me when I get to Japan??

Thanks in advance.

Eki-net cannot be used overseas?

I used to do ticket buying for people, but there is no market because you can select/buy tickets online. Not sure why you can't select seats.
The other companies I am aware of that do ticket buying charge way too much.