I need help working out paypal new smart checkout (no item shop, just add balance)

[MikePHP]

Hello,

This is the code I've been using for a decade:

HTML:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
	<input type="textarea" id="euroAmount" name="amount" value="" placeholder="How much to spend.."/><br/>
	<input type="hidden" name="cmd" value="_xclick"/>
	<input type="hidden" name="business" value="mymail"/>
	<input type="hidden" name="lc" value="en_US"/>
	<input type="hidden" name="item_name" value="Item at mysite"/>
	<?php echo '<input type="hidden"  name="custom" value="'.$user_id.'">'; ?>
	<input type="hidden" name="button_subtype" value="products"/>
	<input type="hidden" name="currency_code" value="EUR"/>
	<input type="hidden" name="no_shipping" value="1"/>
	<input type="hidden" name="no_note" value="1"/>
	<input type="hidden" name="notify_url" value="https://mysite/ipn.php"/>
	<input type="hidden" name="return" value="https://mysite/shop"/>
	<input type="hidden" name="rm" value="0"/>
	<input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynowCC_LG.gif:NonHostedGuest"/>
	<input type="submit" class="btn btn-success" name="submit" value="Purchase with paypal"/><br/>
</form>

PHP:

<?php
if(gethostbyaddr($_SERVER['REMOTE_ADDR']) != 'notify.paypal.com'){ 
	exit();
}
	// Variables //
	$receiverEmail	= htmlspecialchars(strtolower($_REQUEST['receiver_email']));
	$paymentStatus	= htmlspecialchars(strtolower($_REQUEST['payment_status']));
	$account_id		= (int)$_REQUEST['custom'];
	$time			= (int)time();
	$transaction_id	= htmlspecialchars($_REQUEST['txn_id']);
	$transaction_type	= htmlspecialchars($_REQUEST['txn_type']);
	$money_gross	= (int)$_REQUEST['mc_gross'];
	$money_fee		= (int)$_REQUEST['mc_fee'];
	$money_net		= (int)($money_gross-$money_fee);
	$money_currency	= htmlspecialchars(strtolower($_REQUEST['mc_currency']));
	$payer_email	= htmlspecialchars($_REQUEST['payer_email']);
	$payer_id		= htmlspecialchars($_REQUEST['payer_id']);
	$first_name		= htmlspecialchars($_REQUEST['first_name']);
	$last_name		= htmlspecialchars($_REQUEST['last_name']);
	
    // Setup
    $receiver = 'mymail';
    if($paymentStatus == 'completed' && ($receiverEmail == $receiver) && $money_gross > 0 && $money_currency == "eur"){
		//allowed payment, giving the user whatever was ordered
    }elseif($paymentStatus == 'reversed' && ($receiverEmail == $receiver)){
		//do something
    }else{
        exit();
    }
?>

I've tried to look into manuals for the new smart checkout, but to be honest, there's A LOT of things there and since I'm not used to those new things I don't even know where to look.

All I need is a HTML/PHP system there the amount of EUR is changeable depending on how much the user wants to purchase for.

To make this easier to understand, I let users "add balance" to my website.

So that's what I want and need, a friendly code that allow the user to edit how much he'd like to add to balance and that the balance is refilled automatically and quickly.

I'm sorry for sounding like a complete noob, but these new things are so confusing, especially when it seems like most of it is for carts of physical item sellers.

Sincerely, Mike