GoCardless is a direct debit payment merchant which will allow you to take payments for invoices within Clear Books. In the situation where you have existing mandates in your GoCardless account, you are able to link these to existing customers in Clear Books.
Head to Tools > Integrations > GoCardless > Mandates.
Click on the Link Existing Mandates link.
Select the relevant customer you wish to link to the Mandate ID shown on your GoCardless account, then hit Submit.