I'm not sure. The Pending status is supposed to indicate an order that hasn't been paid yet (check or money order payment, for example). So, a Pending order also has a PaymentStatus of "Not Paid". If you let Pending orders through (by adding another case to your if statement), the ones that aren't paid will still be filtered out, but it would let us use Pending to mean something else.
It's your call.
One other small thing you might consider: instead of not showing the button at all, it might be better to show it disabled. When I see an order without the button, I get nervous that the plugin isn't working.
Thanks!