I don't find anything specifically stating this virus affects OS X. It apparently is delivered as a .exe executable. They will not run on OS X, although they will run on Windows.
Your computer cannot open the malware. It requires you to click on a link according to Canada Post's site.
Helpful Links Regarding Flashback Trojan and Virus Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read the FAQ on malware.
A link to a great User Tip about the trojan: Flashback Trojan User Tip.
To check for the trojan: Anti Flashback Trojan 2.0.4.
A Google search can reveal a variety of alternatives on how the remove the trojan should your computer get infected. This can get you started. Or the preferred method is to use Apple's protection tool: Flashback Malware Removal Tool 1.0.
Or, open Software Update. If you do not have the Apple protection software installed it will download and install it via Software Update. If no update appears that means you either already have it installed or it isn't needed for your system. The software is only available for Leopard, Snow Leopard, and Lion versions of OS X.
Also see Apple's article About Flashback malware.
For general anti-virus protection I only recommend using ClamXav.