The javascript allows only number and decimal points:
<SCRIPT language=Javascript>
<!--
function isNumberKey(evt)
{
var charCode = (evt.which) ? evt.which : event.keyCode;
if (charCode != 46 && charCode > 31
&& (charCode < 48 || charCode > 57))
return false;
return true;
}
//-->
</SCRIPT>
The blank amount to pay field displays the $ until the field is active:
<input type="num" name="charge_total" value="$" onfocus="this.value=''" onkeypress="return isNumberKey(event)" >