Here is what I do.

First follow the link from the e-mail

At the box labled 評価： You need to choose 1-5 
options  1 is the best praise 5 is the worst 
praise 

-if you are happy with the transaction pick 

非常に良い 

if you are undecided pick

-どちらでもない

-if you are unhappy pick

非常に悪い 


In the larger box labeled コメント：

Just write in a comment.  Thank you arigatou etc.

I write スムーズに取り引きできました。  Which 
basically says the deal went smoothly.
Hope this helps.

Kaze is formless.
If you have more questions post or e-mail