Scan USPS barcode from a Mac

Delicious Monster makes an awesome piece of software called Delicious Library that's intended to do exactly what you're after: scan and catalog your library (of books and other things). And it all works through your iMacs built-in iSight camera. And it works very, very well. From their website:

“Wait, I just hold a CD or DVD or video game or book or whatever up to my webcam, and it magically reads the UPC and downloads that item’s cover and all pertinent information about it, and displays all my stuff on photorealistic shelves? I’ll take it! Right now! This is why I bought a computer in the first place! ”

Yea, it's very cool to watch it in action. Very easy to use.

Apple's Image Capture app is intentionally minimalist. It doesn't have significant Preference settings, and no user-friendly way to turn off the overview scan.

In theory, anything in OS X is hackable, given sufficient knowledge and time. In practice, you should consider a 3rd party scanner app instead.