You, as an expert, provides a service to the client. In case the client requires an invoice, it is up to you, the expert providing the service, to send it.
We, Peerwith, are providing the platform to facilitate the connection and transaction. For that we charge a service fee, which we invoice to you, the expert! So that is why we send the invoice, we have to for tax reasons.
Note that our invoices are for the service fee only, not the full amount. However, the full amount is mentioned, because it is relevant for the pay out process. You never pay our invoices, we will simply deduct the service fee from the amount that we pay out to you.