How to secure paypals purchase forms

How to secure paypals purchase forms

I'm trying to help someone with a problem, where you can donate for some
points, but you the source code will allow you to edit the values of the
paypal button. For instance, i choose 5 points which should cost 5 dollar,
but when I edit source code I can see the whole paypal form with values,
and edit the values. So how can I secure the form so that it cannot be
edited?
Here's the code:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="at"
value="nBWAy2wwJn1O1LdoLcMn8AOdPxBBUxhnZfqFvAlGQq1oDm0czxn5FBcpgIW">
<input type="hidden" name="business" value="admin@worldofthegods.com">
<input type="hidden" name="item_name" value="DP|1|452160">
<input type="hidden" name="amount" value="1">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="return"
value="http://www.worldofthegods.com/donate.php?action=done">
<input type="hidden" name="cancel_return"
value="http://www.worldofthegods.com/donate.php?action=cancel">
<input type="hidden" name="notify_url"
value="http://www.worldofthegods.com/ipn_donator.php">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="tax" value="0">
<input type="image"
src="http://worldofthegods.com/images/PaypalButton.gif" border="0"
name="submit" alt="Make payments with PayPal - it's fast, free and
secure!">
</form>