you need to view the source of your page and see if the form action includes the folder in the url, if not you will need to make it do that.
Assuming you are already including the folder correctly when building the url for the iframe, then the page in the iframe should post back to the same url with the folder.
something like this might help
try
{
SiteUtils.SetFormAction(Page, Request.RawUrl);
}
catch (MissingMethodException)
{
//this method was introduced in .NET 3.5 SP1
}
Hope that helps,
Joe