When you install Postal with NuGet it replaces System.Web.Razor 2.0 with System.Web.Razor 1.0. I had to delete the reference to System.Web.Razor 1.0 and then added back in System.Web.Razor 2.0. You have to additionally set Copy Local to True in the Properties for System.Web.Razor and then rebuild the project. Once I did that VS2012 copied the System.Web.Razor.dll to the deployed bin directory and the problem cleared.
Postal also adds these dependency's to the web.config. I removed them and it still works properly.
<dependentAssembly>
<assemblyIdentity name="System.Web.Razor" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-1.0.0.0" newVersion="2.0.0.0" />
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="WebGrease" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-1.3.0.0" newVersion="1.3.0.0" />
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="RazorEngine" publicKeyToken="9ee697374c7e744a" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-3.0.8.0" newVersion="3.0.8.0" />
</dependentAssembly>
UPDATE 3/3/13
AD has just released Postal v0.8.2 which updated System.Web.Razor to 2.0.
The WebGrease dependency, with the release of ASP.NET and Web Tools 2012.2 (http://www.asp.net/vnext/overview/fall-2012-update) appears to be the default now.