<div class="invoice">
    <input class="date" />
    <input class="obsolete" />
    <input class="pending" />
</div>
<div class="payment">
    <input class="date" />
    <input class="obsolete" />
    <input class="pending" />
</div>

function initForm($container) {
    $container.find("input.obsolete").click(toggle_checkbox);
    ...
}

$(document).ready(function(){
    initForm($("invoice"));
    initForm($("payment"));
});

// Use plural form to show that we select multiple elements
var $obsoleteInvoices = $('input.invoice-obsolete');