public async Task UpdateCheckout(Checkout checkout)
{
var existing = await Checkouts.SingleAsync(c => c.CheckoutId == checkout.CheckoutId && c.TerritoryId == checkout.TerritoryId);
Checkouts.Remove(existing);
Checkouts.Add(checkout);
await SaveChangesAsync();
}
public void AddItem(ICheckoutable newItem)
{
Checkouts.Add(newItem);
}