The "Redirect to confirmation page" option of the Ecommerce Checkout widget does not work properly in a multisite environment where the site URL is a subfolder and not a root domain.

For more information about Bug 219342, see https://feedback.telerik.com/Project/153/Feedback/Details/219342.

Until the fix is available, use the workaround below. 

1: Customize the Checkout widget, override the InitializeControls method and redirect to the proper URL
 

using System;
using System.Linq;
using System.Threading;
using System.Web;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Ecommerce.Orders.Model;
using Telerik.Sitefinity.Localization.UrlLocalizationStrategies;
using Telerik.Sitefinity.Modules.Ecommerce.Orders.Web.UI;
using Telerik.Sitefinity.Modules.Pages;
using Telerik.Sitefinity.Multisite;
using Telerik.Sitefinity.Pages.Model;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web.UI;

namespace SitefinityWebApp
{
    public class CustomCheckout : Checkout
    {
        protected override void InitializeControls(GenericContainer container)
        {
            if (this.CheckoutWorker.IsOnConfirmationStep())
            {
                bool shouldRedirectToConfirmation = this.CheckoutWorker.CheckIfConfirmationRedirectHasToBeMade(this.ConfirmationStepPageId);

                if (shouldRedirectToConfirmation)
                {
                    Order order = this.OrdersManager.GetOrders().Single(o => o.OrderNumber == this.ShoppingCart.OrderNumber);
                    this.CleanUp(this.ShoppingCart.Id);
                    string confirmationPageUrl = this.CustomGetConfirmationRedirectUrl(order.Id, this.ConfirmationStepPageId.Value);
                    this.CheckoutWorker.RedirectToConfirmationUrl(confirmationPageUrl, url => this.Page.Response.Redirect(url));
                }
            }

            base.InitializeControls(container);
        }

        public string CustomGetConfirmationRedirectUrl(Guid cartOrderId, Guid stepPageId)
        {
            PageManager pageManager = PageManager.GetManager();
            var urlService = ObjectFactory.Resolve<UrlLocalizationService>();

            PageNode confirmationPageNode = pageManager.GetPageNode(stepPageId);
            string confirmationUrl = urlService.ResolvePageUrl(confirmationPageNode, Thread.CurrentThread.CurrentUICulture);

            var siteUrl = HttpContext.Current.Request.Url.Scheme + "://" +(SystemManager.CurrentContext as MultisiteContext).CurrentSite.LiveUrl;
            confirmationUrl = siteUrl + VirtualPathUtility.ToAbsolute(confirmationUrl);
            confirmationUrl += "/order/" + cartOrderId;

            return confirmationUrl;
        }

        private void CleanUp(Guid cartOrderId)
        {
            var cartOrder = this.OrdersManager.GetCartOrder(cartOrderId);
            this.OrdersManager.DeleteCartOrder(cartOrder);
            this.OrdersManager.SaveChanges();
            this.RemoveShoppingCartCookie();
        }
    }
}

2. Build the project
3. Register the custom class as a custom widget in the Sitefinity Toolbox
4. Drag the custom widget on the page instead of the default one

 

References to Other Documentation:
Sitefinity Documentation, Register a new widget in Sitefinity CMS toolbox http://docs.sitefinity.com/register-a-new-widget-in-sitefinity-toolbox