You can print out a confirmation in the enrollment system or ask for a signed one, which will be sent to you by post. See Step 4 in the guide here: https://amu.edu.pl/en/admissions/full-study-programs-online-enrollment-system
Details for paying the tuition fee may be seen in USOS. To enter this system, do the following:
When you open your applicant account on https://rekrutacja.amu.edu.pl/, you should see the details of your student account. You are supposed to set a password for your student email (the one that ends with @st.amu.edu.pl). Once you have set a password, you can access USOSweb and Microsoft Teams using the same student email and password. Then go to https://usosweb.amu.edu.pl/kontroler.php?_action=news%2Fdefault&lang=en.
You can also find information in the students’ intranet, to which you’ll find a link (besides other useful information) on this site: https://amu.edu.pl/en/education
Also check out the Welcome Center:
https://amu.edu.pl/en/main-page/welcome-center