To be honest, the code you have is attempting to do too much in a none-WPF recommended way. If I were you, I would look to use binding on your form so that you can simplify your code. That's your first hint - look at how to convert this to use binding.
If you must use the code the way you have it, you might want something like this:
string purpose = cbxPurpose.SelectedItem?.ToString();
string compartment = cbxCompartment.SelectedItem?.ToString();
if (compartment == "Truck Full" && purpose == "Leaving Finished Product")
{
}
As there's no indication where TopSeal or BottomSeal could possibly come from, I can't help any more I'm afraid. As OriginalGriff says, we can't see your screen so if you don't provide full information, our ability to help will be limited.