Ah hah!
Well, the online version wouldn't be hard at all to replicate in as a Mac app (I'm a Mac developer). Why wouldn't you use their online service though? Is it missing some functionality you need? If so, can you explain what you'd need?
Also, you may have noticed from my signature that I already have a barcode app, so I could roll that code into a label maker pretty easily if needed.
Sounds like a fun project. If you'd like to discuss it in private, send me your email by Private Message.
Rob