Phppointofsale is an excellent POS system. [url removed, login to view]
But we need more functions integrated to it.
1. User can generate sales quotations.
2. User can convert quotations into and Job Order (Just a list detailing the jobs)
3. User can convert the quotations to invoices
4. User can create invoices
5. Very Simple reports
Implement these with as little page loads as possible. eg use AJAX
Use back user and customer data.
When a new version of phppointofsale is available, it should be upgradable with little changes to your code.
More details in the attached docs.