When I view source of your page, the add to cart button forms are very inconsistent.
Some have 2 form tags;
<form name="eCart1_5_ATC_1" method="POST" action="/heights.php">
<form name="eCart1_3_ATC_1323" method="POST" action="/heights.php">
<input type="hidden" name="eCart1_3_ID_Add" value="1427" >
<input type="hidden" name="eCart1_3_Name_Add" value="Rotonda Heights" >
<input type="hidden" name="eCart1_3_Description_Add" value="Reserve Lot 1427 with refundable deposit" >
<input type="hidden" name="eCart1_3_Quantity_Add" value="1" >
<input type="hidden" name="eCart1_3_Price_Add" value="638.19" >
<input type="image" src="images/buynow.gif" border="0" value="Add to Cart" name="eCart1_3_ATC">
</form>
some are correct and only have 1 form tag:
<form name="eCart1_3_ATC_1323" method="POST" action="/heights.php">
<input type="hidden" name="eCart1_3_ID_Add" value="406" >
<input type="hidden" name="eCart1_3_Name_Add" value="Rotonda Heights" >
<input type="hidden" name="eCart1_3_Description_Add" value="Reserve Lot 406 with refundable deposit" >
<input type="hidden" name="eCart1_3_Quantity_Add" value="1" >
<input type="hidden" name="eCart1_3_Price_Add" value="638.19" >
<input type="image" src="images/buynow.gif" border="0" value="Add to Cart" name="eCart1_3_ATC">
</form>
and others edo not have any form tags:
<script language="javascript" type="text/javascript">
<!--
function EJEJC_lc(th) { return false; }
// -->
</script>
<input type="hidden" name="eCart1_3_ID_Add" value="1445" >
<input type="hidden" name="eCart1_3_Name_Add" value="Rotonda Heights" >
<input type="hidden" name="eCart1_3_Description_Add" value="Reserve Lot 1445 with refundable deposit" >
<input type="hidden" name="eCart1_3_Quantity_Add" value="1" >
<input type="hidden" name="eCart1_3_Price_Add" value="638.19" > <input type="image" src="images/buynow.gif" border="0" value="Add to Cart" name="eCart1_3_ATC">
you will need to correct these inconsistencies so that each form on the page has only one opening and closing form tag.