I am trying to execute some custom code after a user upgrades to a paying plan in Prosite's (i.e. remove them from an email reminder campaign that they are on a trial plan which will expire, along with other helpful tips about their site). Are there any hooks in Prosite that I can use to kick off my custom code after a user selects a paying plan? Is the psts_upgrade hook the right thing to use? Or is there a hook that executes somewhere else after a successful payment?