@hohai610
I think Daniel's advice is the most appropriate. Don't start over with a new effort from scratch. Find an existing application that does what you need and later, if you decide it's not adequate, then you can start designing a custom relational database application from a position of knowledge.
That said, the problem you describe is the result of an inappropriate table design--which is a very common mistake made by people new to using relational database application development software like Access.
Three fields in a table called, "UPC,UPC1,UPC2" are what is often called a Spreadsheet Style table. It comes from trying to create an Access table in the same way you would an Excel spreadsheet. It's called a "Repeating Group". Many, many people make this mistake so don't feel bad about that, but do invest enough time in learning how relational database applications work, and how they should be designed.
Later, when you've had enough experience with a properly designed POS system, you can start over designing your own more valid relational database application and get a useful result. After all, your objective is, I assume, running a business, not becoming a database developer.
