A gizmo is a piece of equipment or technology, that you are not familiar with. If you don't know what it is called - you can call it a gizmo, or several other made-up names, such as doodad, or doohickey. A gadget is similar, but usually not as clever as a gizmo. In this case, I don't think it was a barcode scanner, but in fact a signature digitiser.

A gizmo is what we call a gadget when we don't know its actual name. In this case, it refers to a bar code scanner the postman uses to record delivery of the package.