Adding/Subtracting inventory count using barcode scanner

I'm trying to set up an inventory management system and I need some help. My plan is to use a bar code scanner to scan custom bar codes as product comes in and goes out, and have this automatically update the count for that product in an Excel workbook. Is there a way to do this without having a separate button on excel to enter the quantity for that item? We will be scanning items in another part of the building, away from the computer.

I'm attaching the rough, basic workbook I have so far. Column A will be the custom bar codes that will be printed off and hung next to the product. I hope that if we scan that code multiple times using an "Add" function, it will update the quantity in Column C, as well as if we use a "Remove" function, it will change the count based on how many times we scan that code. Any help will be much appreciated!