I have the following interfaces defined in my Domain project, where should I put the implementations like CreditCardPayment, DHLShipping, etc…

Additionally, should I define an enum or register the implementations as keyed services for persistance purposes (store as and convert from string identifier).

public interface IShippingMethod
{
    void CalculateShippingCost(Order order);
    void TrackShipment(Shipment shipment);
}

public interface IPaymentMethod
{
    bool ProcessPayment(Order order, PaymentInfo paymentInfo);
    bool RefundPayment(Order order, PaymentInfo paymentInfo);
}

Asking for an opinion