<form name="autosubmit" target="_new" action="https://select.worldpay.com/wcc/purchase" method=POST>
<input type=hidden name="instId" value="As quoted in your Integration Pack"> 
<input type=hidden name="cartId" value="%webid%"> 
<input type=hidden name="amount" value="%total%">
<input type=hidden name="currency" value="GBP">
<input type=hidden name="desc" value="%webid%">
<input type=submit value="Buy This">
</form>