When a payment comes in we compare payment reference, payment amount and payee to the outstanding invoices and we assign a score to it. If the score is a "perfect match" then we auto-match the payment to the invoice.
For this to happen, the payment reference and the amount needs to be exactly the same as the invoice.
Please note, invoices paid by your customers using the Pay by Bank links option on the invoice (QR code/’Pay invoice’ button) will auto-match the payment for you as well as marking the associated invoice as ‘Paid’ within the Mettle app.