worldpay

We are using WorldPay to process payments for a tiered membership system, for which the payment amount varies dependent upon the membership tier selected. The payment is passed to WorldPay via a form post from a number of hidden fields, including: <input type="hidden" name="amount" value="295.00" /> Essentially, the form is submitted via POST to WorldPay and the user follows a number of steps to process their payment. Once complete, the user is redirected to a specified confirmation page. This appears to be the typical manner in which WorldPay accepts payments. There's an obvious issue here,