rpress_payment_user_id

This filter is applied to use the payment user id, it is including two parameters.
$this->user_id is to get user id.
$this->ID is used to get id.

An example of how you can append additional information:
function get_user_id( $user_id ){
//return payment usr id.
return $user_id;
}
add_filter( ‘rpress_payment_user_id’, ‘get_user_id’, 10,2 );