Arguments on hook | WordPress.org

$submit_button_text = apply_filters('ppress_checkout_order_button_text', sprintf('Pay %s', ppress_display_amount($order_total)), $order_total);

if ($plan->is_recurring()) $submit_button_text = esc_html__('Subscribe', 'wp-user-avatar');

if ($plan->has_free_trial()) $submit_button_text = esc_html__('Start Trial', 'wp-user-avatar');

$submit_button_text = apply_filters('ppress_checkout_order_button_text', $submit_button_text, $order_total, $plan);

This will be fixed in the next update. We’ve added $plan to the other filter where it is missing.

Thanks for pointing this out.