Invoice has been delivered according to Yuki but customer has not received an invoice

In the Sales functionality in the activity log you can check whether the invoice has actually been delivered. When the status is 'Delivered', this means that the invoice has actually been sent via Peppol.

This also means that the invoice on Peppol is ready to be received by the customer's Access Point.

For a detailed description see article Activity log invoices.

Contact the customer to find out if the invoice may not be received because there is a problem with (the software of) the Access Point.