I got the English version of this page , clicked on 'Add to Order' ('Zur Bestellung hinzufügen' in your case) , and entered my 'Contact Information' .
As 'Payment Method' , I chose 'Credit Card' :
I entered my 'Credit Card Details' correctly . But when I click on 'Complete Order' , I always get this error-message :
"You must be logged in to perform test orders, or make sure you've chosen 'Test Payment' as your payment method."
That's not really a choice : when I choose 'Test Payment' I also must be logged in (enter 'Username' and 'Password') .
And there's no 'Log in' button on that page !