Protect paypal buyNow button from changes by JS?

I've never used paypal before so my question maybe stupid. I created paypal button like this:

    <form action="<?php echo $payNowButtonUrl; ?>" method="post" target="_blank">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="business" value="<?php echo $receiverEmail; ?>">

<?php for ($i = 0; $i < count($payment); $i++): ?>
    <input type="hidden" name="item_name_<?= $i + 1 ?>"
           value="<?php echo $payment[$i]['name'] . ' ' . $payment[$i]['licence']; ?>">
    <input type="hidden" name="quantity_<?= $i + 1 ?>"
           value="<?php echo $payment[$i]['qty']; ?>">
    <input type="hidden" name="amount_<?= $i + 1 ?>"
           value="<?php echo $payment[$i]['money']; ?>">
<?php endfor; ?>
<input type="hidden" name="tax_cart" value="10">
<input type="hidden" name="return" value="<?= $returnUrl; ?>">
<input type="hidden" name="custom" value='<?= json_encode($customData); ?>'>
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-BuyNowBF">

<button class="btn" type="submit">buyNow</button>

And i really confused that all varible in form could be changed by js. I tried to make payment in sandbox.paylpal.com with changed params and it make success. How could i protect from this? Or what i'm doing wrong?