Dont try to re-invent the wheel.
Your first step should be producing a flow chart of the existing paper system, and then ask the firms accountant what systems they can work with.
Then do an internet search for suitable software, most companies e-commerce software should be compatible with Sage and that is what most accountants will work with. Sage also do small business software.
Think about using an e-commerce program like Khaos which will do all you want and more. It also has built in user controls that allows you to control access to various part of the program and its functions. This enables you to set a user to accept orders but not to issue card refunds for example.
The advantage is you will have minimal manual writing to do as most of it will be built in.
You would still have to back up the programs database to an disk and take it home and put it in the garage! Or back it up electronically to a remote server. Every day!