Push notifications for chat server

[Matt]

I've written an app that works as a chat client for a particular chat
server software. People who buy this app will have their own copy of
the chat server software running on their own web servers (not mine).

I've had requests to implement push notifications for chat requests.
This is so the user can exit the application, but get a notification
if someone wants to chat, then fire up the app to chat to them. Good
idea.

The problem I have is in figuring out how to implement this.

Because they're all running their own chat servers, I could give them
some server-side code that would handle the push notifications to
Apple's APNS, but I think that means they would each need their own
Apple certificate. Is that right ? I assume that I shouldn't be
giving them a copy of my own certificate.

Another option might be to use Urban Airship and have the chat server
send Urban Airship a message to send an push notification on it's
behalf, but I don't think Urban Airship has that sort of an API does
it ? Also UA's costs might become an issue if I eventually go over
the 1m per month mark.

Are there any other options ?

Am I making this all too hard and there's an easier way ?

This is the first time I've implemented APNS, so any help would be
greatly appreciated.

Thanks,

Matt.