Dynamic ranges

Hi all. I have a situation where i work for a small mart. They have a copy of office 07 installed and i want to manage their invoicing through excel. The way i am doing now is to create a master inventory sheet which gets updated each time a new item gets purchased. But the person operating is not too computer literate and does some mistake from time to time. The workbook has two sheet where on the first sheet is the format of the invoice with formulas picking the item and customer info from the 2nd sheet. I attached a sample file to help