When you using the
using (Html.BeginForm("TestAction", "CombineYourPurchaseBlock", FormMethod.Post))
we will instruct the ASP.NET Mvc framework to find the CombineYourPurchaseBlock-controller and post to the TestAction. If you instead send in null as the controller name the post will go to the current url instead.
Adding post actions into block-controller need to go directly to the controller and then a redirect need to be executed for the visitor, the reason is that you otherwise need to have the same action in the page-controller and in all other block-controllers, otherwise you will get the same exception as in Exception from block when POST is used where the method of POST not is found.