Hier komt een output:
<p id="messageArea" ></p>

<button onclick="window.top.postMessage(JSON.stringify({ 
command: 'Cart', 
method: 'get', 
version: '1.0' 
}),'https://sell1.oliverpos.com');">Stuur</button>

<script>

       let json = JSON.stringify({ 
command: 'Callback', 
version: 1.0, 
method: 'POST', 
data:[ 
{ callBackCommand : 'Checkout', 
hookName: 'checkoutStarted' 
}, 
{ 
callBackCommand : 'Checkout', 
hookName: 'checkoutComplete' 
}, 
{ 
callBackCommand : 'Checkout', 
hookName: 'checkoutSyncComplete' 
} ] 
});

window.top.postMessage(json, 'https://sell1.oliverpos.com');
    	
    	
var messageArea = document.querySelector("#messageArea");
    	
function onMessageHandler(event) {
    console.log(event.data);
  if (typeof event.data === "string") {
    messageArea.innerHTML = JSON.parse(event.data);
  }
}

window.addEventListener("message", onMessageHandler);

</script>