There is currently no option to select a default payment method by customer or to enable to Pay Online button for select customers. We have some customers who would like to pay by "Pay Online Button".
Unfortunately we need to enable/disable this feature as required or have this on all invoices.
We would like to automize this in reapeat invoices.
Hope you can include this feature soon, as it is very tedious to enable/disable for every customer that wants a different payment method.