You can pay using S€PA eBanking (direct debit) or SOFORT Überweisung. These options are only available as one-time-payments, so your PLUS subscription won't automatically renew.
If you experience technical difficulties during the payment process, I suggest updating your browser and make sure cookies are enabled.
For an overview of your PLUS transactions, click your profile picture (top right), open the menu 'Upgrade to PLUS' or 'PLUS Member' and select 'Transaction history'.