Again:
1. How to create user accounts?
2. How to allow user to view past invoices?
3. How can the user pay for invoice online?
4. How to integrate these invoices with Quickbooks.
An example of the process can be seen on [links removed]
]]>https://www.ads-software.com/plugins/search/client+portal
https://www.google.com/search?q=wordpress+client+portal+plugin