E-invoicing, which was initially only for the Public Administration, became mandatory for all invoices between private companies and entities since January 1st, 2019.
Official documentation, including technical rules, is available in the official website of the Italian Revenue Agency.
Developers Italia provides communication channels and collaboration tools for developers in order to help them integrate it in their software. The forum and the Slack channel are not official helpdesk channels because they are not monitored by the Revenue Agency or Sogei.
SDK for PHP
Non-official SDK (maintained by the community) for sending e-invoices from PHP applications
Tool for Python
Non-official tool (maintained by the community) for sending e-invoices from Python applications
Example PHP application
Non-official example application (maintained by the community) for handling electronic invoicing
Test Exchange System (SDI)
Non-official PHP application (maintained by the community) for simulating an Exchange System (SDI) and testing client implementations