Best place to put optional Sign in/Sign up box on a website with auto-register user

We've wrote a website so that users can purchase ticket (for concerts). For users convenience we wrote user temporary registration and auto login systems. So users don't need to Sign in/Sign up to purchase ticket.

For more features(e.g tracking his/her purchase) sign up/sign in is needed, but I don't know where is the best place to put Sign in/Sign up box on the website. I think if I put such a box in header section(e.g) newbie user will think sign up is mandatory.