<tbody>
<tr>
<td>
<input type="radio" name="payment_method" value="bank_transfer" id="bank_transfer">
</td>
<td>
<label for="bank_transfer">Banka Havalesi/EFT</label>
</td>
</tr>
<tr>
<td>
<input type="radio" name="payment_method" value="cod" id="cod" checked="checked">
</td>
<td>
<label for="cod">Kapıda Nakit Ödeme</label>
<div class="altyazi" style="display: block">
<p>Teslimat anında ürün bedeli nakit olarak kargo firması tarafından tahsil edilecektir.</p>
</div>
</td>
<td></td>
</tr>
</tbody>

<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
<script>
$("input[name='payment_method']").on('change', function(){
    if ( $(this).val() == "cod" ) {
        $(".altyazi").css("display","block");
    } else {
        $(".altyazi").css("display","none");
    }
});
</script>