Being able to manage customers or at least import/connect to an addressbook like Google Mail i.e.
This can already be done.
First check out the database with phpmyadmin or sqlbuddy or similar to see what headings there are.
Export your address book as CSV, open it in excel to delete non-used columns and add headings.
Import into the database. Easy!
Whole process would take a few minutes, but it's a once-off. Plus you can import a massive address book and it won't take any longer.
Txopi David Rdgz Taubmann commented
I certainly would recommend this, but under a common standard, like vcf or csv like in gmail contacts does.
in my opinion managing customers is a must have for this kind of system. A great feature to give insight in the needs of a old costumer that has returned a year later for instance.
perfect would be a checkmark if this invoice should be saved as a template, with customer data, taxes, legal info, item description, price per item, but not the amount.
This template should be usable in both normal and recurring invoices.