In order for the Paypal window to open without getting blocked, I am using an anchor with target=_blank.

That's one option, but as long as you call window.open from within the handler for a user-generated event (like click), you can open pop-up windows. So just make sure you call window.open from within a click handler on the link (and then you can close it). Modern pop-up blockers (anything from the last several years) block pop-ups that aren't triggered by a user event, but allow ones that are.

Live example | source:

HTML:

<p><a href=# id=target>Click to open popup</a>; it will close automatically after five seconds.</p>

JavaScript:

(function() {



  document.getElementById(target).onclick = function() {

    var wnd = window.open(http://stackoverflow.com);

    setTimeout(function() {

      wnd.close();

    }, 5000);

    return false;

  };



})();