Applying for China visa in Japan, I'm American

where in Japan are you actually living in? Check first which consular district you are living in and go to obtain the visa from that embassy or consulate. FYI: https://www.travelchinaguide.com/embassy/japan.htm
I'm afraid post application is not available. If you cannot appear in person, you can entrust someone else to submit the documents for you.
To apply a China work visa in Japan, you need to have Japan resident permit. Otherwise, you need to go back to your home country to apply for the visa.