We require that all users fill out the DNN registration including custom required fields. To bypass the Smith Cart registration page I added this to the top of the RegistrationCart.ascx.

<%@ control language="C#" autoeventwireup="true" inherits="Smith.DNN.Modules.BuyNow.RegistrationCart, App_Web_registrationcart.ascx.54f2e0e1" %>
<%
Response.Redirect("~/register.aspx?returnurl=login.aspx");
%>

It would be nice to have a Smith Cart option to use DotNetNuke.Common.Globals.NavigateURL(PortalSettings.RegisterTabId) to force DNN registration.