doesn’t support multiple purchase plans

Example with the <select>:

<select id="licenses">
   <option value="1" selected="selected">Single Site License</option>
   <option value="unlimited">Unlimited Sites License</option>
</select>
<button id="purchase">Buy Button</button>
<script src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<script src="https://checkout.freemius.com/checkout.min.js"></script>
<script>
    var handler = FS.Checkout.configure({
        plugin_id:  '1...9',
        plan_id:    '1...8',
        public_key: 'pk_89...122c',
        image:      'https://your-plugin-site.com/logo-100x100.png'
    });
    
    $('#purchase').on('click', function (e) {
        handler.open({
            name     : '...',
            licenses : $('#licenses').val(),
            // You can consume the response for after purchase logic.
            success  : function (response) {
                // alert(response.user.email);
            }
        });
        e.preventDefault();
    });
</script>