The MVC5 project references MVC 5.1.0.0 in the csproj file, instead of the 5.1.2 version referenced everywhere else including project requirements. This means that NuGet downloads MVC 5.1.2 but then the code itself looks for MVC 5.1.0, and if 5.1.0 was not installed then the search fails and Postal crashes the application.
This is a 1 second fix, if anyone here is still working on this.