You can use invoices with Pay per post system & Membership system. Invoice settings let you to modify Invoice number prefix and subtext.
![Screen-Shot-2015-10-09-at-14.19.20](https://pointfinderdocs.wethemes.com/wp-content/uploads/2019/05/Screen-Shot-2015-10-09-at-14.19.20.png)
You can use invoices with Pay per post system & Membership system. Invoice settings let you to modify Invoice number prefix and subtext.