如果我的paypal购物车总额超过99.00美元，如何添加代码以获得19.99美元的折扣？
<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="hnsabc@mail.com">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="item_name" value="Apex Creatine .47lbs">
<input type="hidden" name="item_number" value="91037 97366 Promo">
<input type="hidden" name="amount" value="35.99">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="button_subtype" value="services">
<input type="hidden" name="no_note" value="0">