Option1.cs

// All service have access to an ISessionManager that persists
// the current ids for the session (ISessionManager interface 
// defined in core, with implementation in web project to 
// store values in session / cookies OOTB)

OrderService.Get/SetCurrentOrder();
CurrencyService.Get/SetCurrentCurrency();
CountryService.Get/SetCurrentPaymentCountry();
CountryService.Get/SetCurrentPaymentRegion();
CountryService.Get/SetCurrentShippingCountry();
CountryService.Get/SetCurrentShippingRegion();
PaymentMethodService.Get/SetCurrentPaymentMethod();
ShippingMethodService.Get/SetCurrentShippingMethod();

// Example
var order = OrderService.GetCurrentOrder(storeId);
order.Items.AddOrUpdate(...);
OrderService.SaveOrder(order);

// Concerns
"Current" setting/getting is split across multiple services so 
requires accessing multiple end points. More confusing?

Option2.cs

// Services are ISessionManager un-aware and instead
// the SessionManager stores and accesses session
// related entities. SessionManager then become the
// interface to get/set all session related entities

SessionManager.Get/SetCurrentOrder();
SessionManager.Get/SetCurrentCurrency();
SessionManager.Get/SetCurrentPaymentCountry();
SessionManager.Get/SetCurrentPaymentRegion();
SessionManager.Get/SetCurrentShippingCountry();
SessionManager.Get/SetCurrentShippingRegion();
SessionManager.Get/SetCurrentPaymentMethod();
SessionManager.Get/SetCurrentShippingMethod();

// Example
var order = SessionManager.GetCurrentOrder(storeId); // Get via session manager
order.Items.AddOrUpdate(...);
OrderService.SaveOrder(order); // Save via order service

// Concerns
Disparate that you get from SessionManager but you'd still
need to go via the OrderService etc to save any changes back