Paypal custom recurring donation form

HTML

<h3>Support this website and it's author via Paypal:</h3>
<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top" onsubmit="f_d_submit()">
<div><label for="a3">Amount in Euros</label> <input type="number" id="a3" name="a3" type="number" min="1" value="1" style="width: 5em;" onchange="f_d_amount()" required></div>
<div><label for="d_form_duration">Duration</label>
<select id="d_form_duration" onchange="f_d_duration()">
<option value="" selected>until canceled</option>
<option value="1">1 month</option>
<option value="2">2 months</option>
<option value="3">3 months</option>
<option value="4">4 months</option>
<option value="5">5 months</option>
<option value="6">6 months</option>
<option value="7">7 months</option>
<option value="8">8 months</option>
<option value="9">9 months</option>
<option value="10">10 months</option>
<option value="11">11 months</option>
<option value="12">12 months</option>
</select></div>
<input type="hidden" name="src" value="0">
<input type="hidden" name="srt" value="">
<input type="hidden" name="p3" value="1">
<input type="hidden" name="t3" value="M">
<input type="hidden" name="cmd" value="_xclick-subscriptions">
<input type="hidden" name="business" value="1234567abc">  <!-- set your paypal/merchant id here -->
<input type="hidden" name="charset" value="utf-8">
<input type="hidden" name="currency_code" value="EUR"> <!-- EUR for Euro, USD for Dollar -->
<input type="hidden" name="item_name" value="Donation to Firstname Lastname">  <!-- So that it makes sense when looking at transaction records -->
<input type="hidden" name="item_number" value="example.no"> <!-- similar to above -->
<input type="hidden" name="return" value="http://example.com/thanks/"> <!-- Always thank your supporters, they will be taken to this page afterwards -->
<div>Donate €<span id="d_amount"></span> <span id="d_duration"></span>?</div>
<div><button name="submit">Submit</button></div>
</form>
<script>
	'strict';
	function f_d_amount()...