Hm, I had no trouble getting that example URL to stick in your Common Notification URL field. Try force-reloading our Admin panel in your browser -- hold the Ctrl key (on PC) or Shift key (on Mac) while you click Reload -- then try entering the notification URL again.
I see you do have Product Notification URLs set up for each of your products. Those would each receive their own transmission of the full order data, only when that item is purchased, separately from any Common Notification URL. Generally if you need to be notified for every order, the Common URL is preferable to Product URLs, so there won't be near-duplicate submissions for multi-item orders.
I don't quite follow your last paragraph? Is this for a notification URL, or are you also redirecting buyers to a Common Thank-you Page URL or Redirection URL? Notification URLs receive the data by the POST method (like a typical webpage form submission, but in this case coming direct from our server rather than as a user submission), whereas redirections use the GET method (appending a query string with data parameters at the end of your URL when we redirect the buyer there), which latter may not be reliable if the buyer never chooses to access their redirection.
