If an invoice is marked as unpaid in Billogram but your customer claims they have paid it, the customer may have made the payment using the wrong bankgiro or OCR number for the payment. In order for us to help you, we need a receipt of the payment from the payer. Their receipt is available from their internet bank. The receipt should clearly show recipient, sender, date, amount transferred, and reference. E-mail the receipt to firstname.lastname@example.org and state which of your invoices the payment was meant for. Our support will then be able to help you with the next step.
Also note that payments may take up to a day be recorded, which means that if your customer made the payment today, your invoice will be marked as paid tomorrow.