You can rely on a custom property that you set on the basket whenever the user logs in the first time with his basket. That way you know that the basket relies on a user that in some point in time was logged in. So your pricing logic will be consistent if he logs in / out all the time. BTW, you should've already set the customer with the member when he logs in? 

Hope that helps.

Best regards

Morten

Sounds right, but I feel like I'm missing something. What I've tried:

When the user logs in, I mark the order with the umbraco member id - if user already have a basket I'll retrieve it (error at basket.Save())

var membershipUser = Membership.GetUser();
if (membershipUser!=null)
{
    bool uCommerceMemberExists = Customer.Exists(c => c.MemberId == membershipUser.ProviderUserKey.ToString());
    if (!TransactionLibrary.HasBasket() && !uCommerceMemberExists)
    {
        PurchaseOrder basket = SiteContext.Current.OrderContext.GetBasket(true).PurchaseOrder;
        Customer customer = new Customer
        {
            MemberId = membershipUser.ProviderUserKey.ToString(),
        };
        basket.Customer = customer;
        basket.Save();
    }
    else
    {
        PurchaseOrder basket = PurchaseOrder.All().Where(x => x.Customer.MemberId == membershipUser.ProviderUserKey.ToString() && x.OrderStatus.OrderStatusId == 0).FirstOrDefault();
        basket.Save(); // <-- Object reference not set to an instance of an object
    }
}

Correction to this line:

PurchaseOrder basket =PurchaseOrder.All().Where(x => x.Customer.MemberId== membershipUser.ProviderUserKey.ToString()&& x.OrderStatus.OrderStatusId == 0).FirstOrDefault();

Should be ("1" instead of "0"):

PurchaseOrder basket =PurchaseOrder.All().Where(x => x.Customer.MemberId== membershipUser.ProviderUserKey.ToString()&& x.OrderStatus.OrderStatusId==1).FirstOrDefault();

This solves the error. I then thought I would be able to retrieve the basket in the basket-template like this:

PurchaseOrder basket =SiteContext.Current.OrderContext.GetBasket(true).PurchaseOrder;

But this gives me the wrong basket - not the one retrieved on log in.

Found this article, but it seems outdated, so I'm trying to get the code up to date:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;
using Umbraco.Core;
using UCommerce.EntitiesV2;
using UCommerce.Runtime;
using UCommerce.Catalog;
using UCommerce.Transactions;
using UCommerce.Infrastructure;
using UCommerce.Api;

namespace AdvancedBasket
{
    public class CustomOrderContext : OrderContext
    {
        protected ICatalogContext CatalogContext { get; set; }

        public CustomOrderContext(ICatalogContext catalogContext, IRepository<PurchaseOrder> orderRepository)
            : base(catalogContext, orderRepository)
        {
            CatalogContext = catalogContext;
        }

        public override Basket GetBasket(bool create)
        {
            // Member is not logged on use default behavior
            if (!HttpContext.Current.User.Identity.IsAuthenticated) return base.GetBasket(create);

            // Otherwise try and load a basket for the current member, create one if it doesn't exist
            Basket basket = GetBasketForCurrentMember() ?? CreateBasket();

            return basket;

        }
        public override Basket GetBasket()
        {
            return GetBasket(false);
        }

        private Basket GetBasketForCurrentMember()
        {
            var member = ApplicationContext.Current.Services.MemberService.GetByUsername(HttpContext.Current.User.Identity.Name);

            PurchaseOrder order = PurchaseOrder.SingleOrDefault(
                x => x.OrderProperties.Where(y => y.Order.OrderId == x.OrderId && y.Key == "MemberId" && y.Value == member.Id.ToString()).Count() > 0
                    && x.OrderStatus.OrderStatusId == 1); 

            if (order != null) return new Basket(order);

            return null;
        }

        private Basket CreateBasket()
        {
            var catalogId = 0;
            int.TryParse(CatalogContext.CurrentCatalog.Name, out catalogId);

            var catalog = ProductCatalog.SingleOrDefault(c => c.Name.Equals(CatalogContext.CurrentCatalog.Name)) ??
                ProductCatalog.SingleOrDefault(c => c.ProductCatalogId == catalogId);

            if (catalog == null)
                throw new InvalidOperationException("Cannot create basket when not in product catalog. Make sure that SiteContext.Current.CatalogContext.CurrentCatalog.Name contains the name of the current product catalog.");

            return CreateBasket(catalog.PriceGroup.Currency);
        }

        private Basket CreateBasket(Currency currency)
        {
            var member = ApplicationContext.Current.Services.MemberService.GetByUsername(HttpContext.Current.User.Identity.Name);

            var catalogGroup = ProductCatalogGroup.SingleOrDefault(g => g.Name.Equals(CatalogContext.CurrentCatalogGroup.Name));

            if (catalogGroup == null)
                throw new InvalidOperationException("Cannot create basket without product catalog group. Make sure that SiteContext.Current.CatalogContext.CurrentCatalogGroup.Name contains the name of the current product catalog group.");

            PurchaseOrder order = new PurchaseOrder();
            order.ProductCatalogGroup = catalogGroup;
            order.OrderStatus.OrderStatusId = 1; // Compile-error: "The property or indexer 'UCommerce.EntitiesV2.OrderStatus.OrderStatusId' cannot be used in this context because the set accessor is inaccessible"
            order.BillingCurrency = currency;
            order.BasketId = Guid.NewGuid();
            order.CreatedDate = DateTime.Now;

            order.Save();

            // Set the member id on the order so we can retrieve it later on
            order["MemberId"] = member.Id.ToString();

            return new Basket(order);
        }
    }
}

I'm not able to change the orderstatus id: "The property or indexer 'UCommerce.EntitiesV2.OrderStatus.OrderStatusId' cannot be used in this context because the set accessor is inaccessible"

I tried to add the following just before order.Save() - but then I get "Object reference not set to an instance of an object." at third line.

var newOrderStatus = OrderStatus.Get((int)OrderStatusCode.NewOrder);
var orderService = ObjectFactory.Instance.Resolve<IOrderService>();
orderService.ChangeOrderStatus(order, newOrderStatus);

Where exactly are you getting a null references exception? Can you paste in the stack trace ? 

Awsome :) 

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