line 64, you are binding to the productThumb column of the WADAproducts recordset
$ATC_itemThumbnail = "".$row_WADAproducts['ProductThumb'] ."";// column binding.
the SQL for the WADProducts recordset is at line 47:
$query_WADAproducts = sprintf("SELECT ProductID, ProductName, ProductImage, ProductName, ProductShortDesc, ProductLongDesc, ProductPrice, ProductShortDesc, ProductLongDesc FROM products WHERE ProductID = %s", GetSQLValueString($ParamProductID_WADAproducts, "int"));
the select statement of that recordset does not include a ProductThumb column.
there is ProductImage, but not ProductThumb.
you either need to edit the recordset and add the ProductThumb column to the select statement, or edit the add to cart and use a different column binding for the thumbnail column.