wpinv_form_elements wpinv_form_items wpinv_form_refunded wpinv_form_cancelled wpinv_form_failed wpinv_form_elements _wpinv_custom_id _wpinv_custom_name _wpinv_custom_singular_name _wpinv_vat_rule _wpinv_vat_class _wpinv_type _wpinv_editable _wpinv_dynamic_pricing _wpinv_price _minimum_price _wpinv_is_recurring _wpinv_recurring_period _wpinv_recurring_interval _wpinv_recurring_limit _wpinv_free_trial _wpinv_trial_period _wpinv_trial_interval