You can create additional registration questions in Athletics.app, via the right-hand drop-down menu --> 'Online registration settings' (scroll down far).
There you can add a question, such as:
"Purchase 1st ticket" YES (cost: 5 euros). NO (cost 0 euros).
You could add even more questions so that people can order multiple tickets.
Unfortunately, you cannot (yet) create a dropdown menu and attach costs to it.
This has been automatically translated into English