In order to have easyRates calculate the shipping fee for Japan Post, all of the Shopify products need to have their weight configured.
easyRates is an app for automatically calculating the shipping fee for Japan Post International mail, which requires the weight of the order. In Shopify, the weight of the order is based on the sum of each product's weight so make sure that all your products have the accurate weight configured.
There is also a bell button at the upper right corner of the settings menu for easyRates which will automatically detect if there are any products that do not have their weights configured.