Bring PAY ONLINE out from behind Customer Portal

I have clients to whom I have resold Splashtop Remote Access. This requires a portal user to be created.

Creating that portal user then removes the ability for the business owner or accounting employee to easily pay invoices online. Since one portal user exists, I’ll have to create another one for the business owner/accountant.

It seems to me that it would be easier if they customer could simply click the unique link for that specific invoice and pay without having to login.