Let me ask someone a direct question.
Where's the support email address?
I simply want to know if I can run Airmail on 2 machines for 1 license fee.
Can't find that anywhere.
Tony D commented
To contact support you can click the blue chat icon in the bottom right of the screen on airmailapp.com on a computer. They are very good about replying and helping there. You can also create a support ticket at http://support.airmailapp.com/contact
As for fee's - Airmail purchased via the App store are tied to the account you purchased it with, so if your using the same account on both machines - it should work on both.