I'd go with Access. It has it's own little wizard for invoicing. You can set all your clients up, and enter order's, price codes etc for invoicing. The data entry page is far more presentable and easier to read than a spreadsheet. The reporting is easy, and you can export your data to excel. You can do mail merges etc. It's fairly simple, I taught myself how to use it in about 4 hours. In that time I had set up my invoicing database, costs, customers, the look of my screens and reports.
Kylie