Add Go Cardless button to Sales Invoices

It would be a very good idea to include go cardless payment buttons to sales invoices. The current go cardless payment option is only available for the customer on an html invoice. There needs to be 2 clear payment buttons on PDF invoices

1st button - pay by gocardless (similar to paying by paypal) this just pays a one off invoice
2nd button - set up pre-authorisation/DD

Maybe slit the payment options dropdown box at the bottom of sales invoices, have one saying gocardless payment (for one off payments) and the other gocardless DD (for pre-authorisations) each linked to a button on an invoice.