the where clauase of your recordset is not valid:
WHERE ProductName > '$50.00'
the product name is a text column, you cannot perform math function on text.
Greater than is a math function, one text value cannot be Greater than another text value.
the greater than comparison can only be performed on numbers.
This means your recordset is empty which is causing the add to cart function to fail, since there are no items in the recordset to lookup.