If you are staying at a good hotel, i.e., one with a concierge, the best way would be to contact them and ask them to get tickets for you.

Regarding the website, I know nothing about it but it does say that they buy their tickets at the box office. That's fine if you want one of the less expensive tickets, but no good if you want to sit right down on the main floor in a good seat (A or B box, or ringside). Most of those tickets are not controlled by the box office, they are controlled by sumo "tea houses" which you, or someone acting on your behalf, has to have been introduced to before you can get tickets from them.

If the website is legit, the service charge of Y1200 per ticket isn't that bad for the midrange tickets, although they do add shipping in addition. It is a bit mysterious why shipping costs go up as a function of the value of the tickets you buy since tickets are not very heavy!

I would feel a LOT more comfortable about this site if they had some real contact information on it, like who was running it, an actual address, and a phone number. When you pay thru paypal I would guess it's a lot harder to protest charges if these folks turn out to be fake.

If you go midweek and aren't picky about seats, you can probably buy something yourself on the morning of the match. Alternatively, if the box office is sold out, there are always ticket touts out front of the arena, and I have managed to buy tickets from them without understanding or speaking Japanese. The language of money and sign language is international! There isn't a problem with counterfeit tickets purchased from touts, AFAIK.