It's possible to get details of Practice Manager invoices using the Invoices API endpoints:

https://developer.xero.com/documentation/api/practice-manager-3-1/invoices

However, it's not possible to create or update invoices using the API. It seems to be possible to create and update most other objects using the API so why not invoices?

This functionality would be very useful as invoices are a fundamental part of the system.