The Wordpress Plug-in allows clients to have their own payment form. This payment form is linked with an invoice system. When a client has the plug-in installed on his or her site and a payment has been made, after a correct payment also an invoice is sent to his or her customer by email. Furthermore also a copy is sent to clients email address for administration purposes. But also in the admin section of this plug-in all correct handled payments are displayed in a nice overview, this section is secured by an extra password in WP, so that other potential programmers or helpers of the site cannot see the transactions of the client. This plug-in will create unique payment pages/forms after all settings are saved. You can compare this feature with the Contact Form 7 plugin, where a unique code will be created that can be put on your website, so that the contact form will be shown. Similar feature is asked. More details will be shared after approval.
This looks like big project, but the expert knows it a small project.
Anyone with the right experience and skillset can do this job within a short time. I only need you if you have this experience and knowlegde to work straight forward to completion of this job.