I am interested in the workaround, most clients find it hassle to log in just to pay. It would be nice to make it work similarly to and.co. When a client receives an invoice in their email, they can pay the invoice using the payment link without logging in, especially the invoice pay button have a unique URL parameter.
Sorry, for reviving this thread. I am pretty curios myself, would be happy to pay for it if it can be done.