You could customize the code for the message and use the code in RED:
/includes/modules/order_ttotal/ot_coupon.php
around the line 192:
Code:
// $messageStack->add_session('redemptions', sprintf(TEXT_INVALID_REDEEM_COUPON_MINIMUM, $currencies->format($coupon_result->fields['coupon_minimum_order'])),'caution');
$dc_check = $_POST['dc_redeem_code'];
$dc_link = ' <a href="javascript:couponpopupWindow(\'' . zen_href_link(FILENAME_POPUP_COUPON_HELP, 'cID=' . $coupon_result->fields['coupon_id']) . '\')">' . $dc_check . '</a>';
$messageStack->add_session('redemptions', sprintf(TEXT_INVALID_REDEEM_COUPON_MINIMUM, $currencies->format($coupon_result->fields['coupon_minimum_order'])) . $dc_link, 'caution');
and see if having the popup to explain what the Discount Coupon is about helps ...
You could also change the define in the language file(s) for:
TEXT_INVALID_REDEEM_COUPON_MINIMUM