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Is there a hack that allows you to automatically send a message to someone who just Donated to the forum suing the Paid subscription method? Also is there a mod that pops up when you get a Private message?

I'm not aware about the addon for PM ing user after paid for subscription.
But I can do custom work if you want. PM me.

About the pop up when get a PM, it's already built-in with VB.
This is per user setting and user can set them in their usercp.
But you can set the default in VB options > User Registration Options > Default Registration Options

The sad thing is I don't have money.