Hello,
The installation and configuration is the easiest part. And the Xero part is still my expertise. Considering you need it as a regular scheduled sync, then I will write the code which will read A2Billing database for the invoice part and pass to Xero using their web service as well. this script will be wrapped as a cron-job and set as a monthly execution.
Although I am able to make it synced as any time invoice generated. But, I will let you choose, monthly or event based sync. Whichever you choose I am able to handle between both.
Regards,
Yuldal