Disable coupons field from checkout page when order total amout is less than $500
Hope it works.

function module_form_commerce_checkout_form_checkout_alter(&amp;$form, &amp;$form_state, $form_id) {

  if ($form_state['order']-&gt;commerce_order_total['und'][0]['amount'] &lt; 500) {
    unset($form['commerce_coupon']); // enter coupon form name
  }
}