To customize PHP.INI files for DA servers, there are many, many methods. The one method that works for nearly all handlers and webservers is to configure the php [PATH] or [HOST] sections.
Create a uniquely named .ini file in the directory below, where XX is equal to the PHP version you are using:
cat /usr/local/php73/lib/php.conf.d/domain.com.ini [HOST=domain.com] post_max_size = 128M
Then restart the webserver.
This method works for all webservers. Load the phpinfo page for the site to confirm that the value has been changed.
If you were curious about the varying methods, see the links below: