Invoice issued by customer

Hi,

I sold a print on a Dutch website (Werk Aan De Muur) for which I received a commission on the sale. They sent me an invoice marked ‘invoice issued by customer’. How do I process this in my income? Do I create a sales invoice for this myself in Dexxter? Do I upload it as an existing invoice with separate numbering (e.g. selfbill-001)?

Thanks in advance!