Price calculation fails due to different field name

The default rule shipped with the module assumes a certain field name. But if you name your field differently, it will fail. So, instead of using a default rule, we could just generate the right rule when the field is created and remove it when it is deleted?

For that we can use the module name as machine-name prefix instead of the regular 'rules_' prefix - which is for rules created via the admin UI. That way we can make sure we do not run into any conflicts.