Paypal & Firebug plugin with firefox

How is it that I can encrypt a button such as

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
                          <font size="2">
                          <input type="image" src="https://www.paypal.com//en_US/i/btn/x-click-but02.gif" border="0" name="submit8" alt="Make payments with PayPal - it's fast, free and secure!" />
                          <input type="hidden" name="add" value="1" />
                          <input type="hidden" name="cmd" value="_xclick" />

                          <input type="hidden" name="custom" value="admin" />
                          <input type="hidden" name="business" value="paypal@pimpin187.com" />
                          <input type="hidden" name="item_name" value="Pimpin187 SPECIAL DOUBLE CREDITS  2,600 credits admin" />
                          <input type="hidden" name="item_number" value="1" />
                          <input type="hidden" name="item_id" value="1" />
                          <input type="hidden" name="amount" value="20.00" />
                          <input type="hidden" name="page_style" value="Primary" />
                          <input type="hidden" name="no_shipping" value="1" />
                          <input type="hidden" name="notify_url" value="http://www.pimpin187.com/ipn.php" />

                          <input type="hidden" name="return" value="http://www.pimpin187.com/index.php" />
                          <input type="hidden" name="cancel_return" value="http://www.pimpin187.com/index.php" />
                          <input type="hidden" name="currency_code" value="USD" />
                          <input type="hidden" name="bn" value="PP-ShopCartBF" />
                          </font>
                        </form>

Code (markup):

so that someone cant use plugins such as firebug for firefox to change the amount that it costs to purchase something. I have in my ipn file to check amount and call back from paypal however it still is not secure enough. it still comes back as a bought purchase for the original amount and cost.

Thanks,
TJ

 

any help here how to encrypt it with still holding the custom field as it has a php variable?

 

You have an option in your account.

Login to your account
merchant Tools
Buttons and fill up the required options in and check generate encrypted button.

It'll be fine.

 

but do I just exclude or add the html line for the custom line?

as it pulls a variable within php

thanks again for your help