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

[me2me2]

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.

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.