Hello,
I have work experiences on similar application/portal/system . i have designed invoice systems for many different situations such as retails shops, construction companies, online shopping websites generating dynamic/ automatic invoices using PHP, MySQL and other back end and front end languages.
Now for your situation as i understood:
1. You already have a website and want to add functionalities such as invoice generation, costumer database, product database and user/admin Authentication.
2. You want almost every thing automatic, which is a challenging and time consuming job but i can ensure you full satisfaction.
2. Your budget is a bit low, but i want to do your project because i need it to gain freelancer reputation. I am new here just joined this site few day ago. i am a professional PHP programmer and building application since 2 years now. If you doubt this you can ask me any time to show my works or even check my profile the only contest i did and i am winning it.
I have discussed the progress of the project in milestones...
Thanks
Meeshal k