I would like for clients to be able to pay for specific invoices/statements online via a general "online billpay" page that can be linked to from my website, like these examples:

https://illinoisderm.edoctorlink.com/
https://billpay2.poscorp.com/patientpaymentservices/quickpay/Dermatology%20and%20Aesthetics

The page/function in these examples appears to be powered by a third-party client services software similar to MCP/Jituzu, but is white-label branded with the provider's branding.