Hi from the States.
I opened your Access file - you have a lot of tables.
I'm not sure what you mean by, "simple." But this project could be massive. Billing software can run into thousands of dollars.
My bid is for a SIMPLE billing application that will run under Windows 7, and Mac OS 10.5-10.8.
Hire me with a reasonable expectation. If you tell me what you need, I'll try to accomplish that within your budget.
Thanks Eric