POST data not appearing in CakePHP controller

I'm using AJAX on a knockout.js form to post some information that CakePHP should receive, however, Cake doesn't seem to find anything. Also, the alert isn't appearing despite a 200 status (OK) from the POST.

Here's the AJAX

$.ajax({  
          url: "/orders/finalize_payment",  
          type: "POST",  
          dataType: "json",  
          contentType: "json",  
          data: JSON.stringify({"customer": customer_id}),  
          success: function(){              
            alert("success");  
          }
    }); 

Here's the corresponding action in the orders controller. Right now, I completely stripped it to just the bare minimum.

function finalize_payment($id = null){
    $this->layout = false;
    $this->autoRender = false;
    if($this->request->is('post')){ //the user has submitted which status to view
        print_r($this->request->data);
            echo "test"; //just to make sure it's reaching this point
    }
}

When I open up the network tab in chrome, it shows the request payload as

customer: 1

The POST shows as success, status 200. I checked the response headers and it just shows

array
(
)
test

Despite chrome showing a payload being sent, CakePHP isn't finding it apparently.

Update

I changed the request from AJAX to $.post and it worked. I still have no clue why

$.post("/orders/finalize_payment",{"customer_id":customer_id},function(data){
        alert('success');
 });