Around the time you receive your shipment, the supplier will send you an invoice with a certain payment deadline. You actually have to indicate that you want to pay the invoice by hand, and you can choose to do so at any date, depending on your financial situation. Don't wait too long, however, as a fine has to be paid for late payment.
Paying a bill is quite simple: select the invoice to pay in the top table, and click the 'Pay' button, which will prepare the payment process between your bank and the bank of the company that invoiced you.