How to secure paypals purchase forms - html

Question

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>

Answer 1

In this way, it's impossible. I guess what you want to do, and this is not the correct way you are following.
You need to verify each payment and make sure they are valid and payed in correct amount.
Paypal IPN is the solution and you need some Server-side programming.
Read more:
Paypal API Question - Can I use Return Url to verify payment
How to verify successful payment of paypal

Answer 2

I'd suggest using something like Express Checkout, which allows you to send the transaction details in an API call instead of showing them in HTML code to the buyer.
However, barring that, your next best option is to use Encrypted Website Payments. https://developer.paypal.com/webapps/developer/docs/classic/paypal-payments-standard/integration-guide/encryptedwebpayments/#id08A3I0P017Q