Payment Successful Callback hook or filter

After following wpmudev on Facebook and the mailing list, I finally had my arm twisted enough to get the MarketPress plugin - great work guys, it is much better than the other junk I have tested and I hope to use it everywhere!

So my question, I would like to hook one of my own functions into a successful payment transaction - is there a callback hook or filter I can use for this?

I have had a scan through parts of the plugin code and as I am unfamiliar and new to this plugin, I am yet to find anything.

function my_custom_function() {
// activate hosting account now
echo 'do stuffs here';

Any help will be greatly appreciated!