Letter of Invitation

Hi! We are traveling around Europe for quite a while and then flying into Japan- where we will get out Chinese Visa before heading to China. If we go to a Chinese travel agency in Tokyo, will they help us with our letter of invitation that we need to supply? I am a bit worried as we will be traveling round China on overground and also exiting china this way. is this going to make it difficult for us to obtain a Chinese tourist visa?