vba xmlhttp post form on a website not working - has array names

Different versions of this code has worked for other posts on the same site (www.cricutmachine.com). Why won't it go to the checkout page as it is written below? I want to get the shipping charge there.

I think it might be because I am sending wrong or not enough name/value pairs. Also I am not sure if the name/value pairs are entered correctly since the names are parts of arrays. None of the other forms I submitted had array names.

Code:

---

xmlhttp.Open "POST", "http://www.cricutmachine.com/shopping_cart.php?action=update_product", False
xmlhttp.setRequestHeader "Content-Type", "application/x-www-form-urlencoded"
'I think the line below is the problem
xmlhttp.Send "'cart_quantity[]'=1&'products_id[]'=324"

---

Instead of using the code above, can I navigate to the url https://www.cricutmachine.com/checkout_shipping.php and download the html file? I would want to do all that with xmlhttp. I am already logging in and adding the item to the cart with xmlhttp. I don't know if it is possible to navigate directly to a url with xmlhttp because that would not involve submitting a form.