In an ASP.NET MVC app I use jQuery for posting data on button-click:
....
$.post(\'<%=
If you're doing a full redirect after a post, then why do it with Ajax? You should be able to perform a tradtional POST here and have it successfully redirect.
If you really want an ajax request to go through and still redirect, a very easy and non-intrusive way to do that would be to return a JavascriptResult from your action instead of a RedirectResult:
return JavaScript("window.location = " + returnUrl);