How Secure are non-hosted buttons?

[syberknight]

i'm needing to have a Subscribe button, but where the user can type in the amount. 

the only way i've discovered that this can be done is by setting the button to NOT be saved, & thus become a non-hosted button like the below code.

but i've read that this is a huge security risk, & that concerns me. so can anybody either ease my mind or provide a method of adding the "a3" input field while keeping it secure?

just how INsecure is this method? what could happen & how?

is it just because the email address is exposed?

is there a way to hide the sensitive info in PHP?

note: i know HTML/CSS; sorta JS, & can work with PHP, but am not a programmer. so the easiest/simplest solution is needed.

THANKS!!!

here's the code example...

<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top">
    
    <input type="hidden" name="cmd" value="_xclick-subscriptions">
    <input type="hidden" name="business" value="EmailAddressHere">
    <input type="hidden" name="lc" value="US">
    <input type="hidden" name="item_name" value="Subscription Name">
    <input type="hidden" name="item_number" value="ID">
    <input type="hidden" name="no_note" value="1">
    <input type="hidden" name="no_shipping" value="1">
    <input type="hidden" name="src" value="1">
    <input type="hidden" name="srt" value="0">
    <input type="hidden" name="p3" value="1">
    <input type="hidden" name="t3" value="M">
    <input type="hidden" name="currency_code" value="USD">
    <input type="hidden" name="bn" value="PP-SubscriptionsBF:btn_subscribeCC_LG.gif:NonHosted">

<label>Enter Your Donation Amount</label>
<input type="text" name="a3" maxlength="60">

    <input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_subscribeCC_LG.gif" border="0" name="submit" alt="Giving">
    <img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>

 Cross-Posted Here!