I had the same problem for the last time I visited Japan. In my case, I wanted to reserve Doraemon museum tickets, but I couldn’t find any procedures to reserve them from outside of Japan, so I used the ticket ordering service to make sure my ticket reserved from outside of Japan.
japanticketbuying
I recommend you using this service. Their response was really fast and the staff’s English was so fluent. In addition, the tickets were much cheaper than voyagin. Hope I answered your question.