The SaferPay workflow seems to be similar to the payPal workflow.
You can view the paypal plugin code at: https://github.com/Merchello/Merchello/tree/merchello-dev/Plugin/Payments/PayPal/src/Merchello.Plugin.Payments.PayPal

This is my confirmation method:

    var invoiceService = MerchelloContext.Current.Services.InvoiceService;
    var customerContext = new CustomerContext(UmbracoContext.Current);
    var basket = customerContext.CurrentCustomer.Basket();
    var preparation = basket.SalePreparation();
    var invoice = preparation.PrepareInvoice();
    invoiceService.Save(invoice);
    IPaymentMethod paymentMethod=preparation .GetPaymentMethod();
    var paymentResult = invoice.AuthorizePayment(paymentMethod.Key);
    if (paymentResult.Payment.Result.ExtendedData != null && paymentResult.Payment.Result.ExtendedData.ContainsKey("RedirectUrl"))
    {               
       string orderConfirmUrl = paymentResult.Payment.Result.ExtendedData.GetValue("RedirectUrl");
       //here redirect to orderConfirmUrl 
    }

If you want to send the confirmation notification you can write:

IEnumerable<string> emailS = new[] { preparation.GetBillToAddress().Email };
Notification.Trigger("OrderConfirmation", attempt, emailS);

Payment plugin implementation depends of SaferPay rules.
For example in payPal, when you request a new payment, you have to add a "returnUrl" param for success or not success payment. You can listen the payment notification creating an UmbracoApiController like this:

[PluginController("MerchelloPayPal")]
    public class PayPalApiController : UmbracoApiController
    {
        [HttpGet]
        public HttpResponseMessage SuccessPayment(Guid invoiceKey, Guid paymentKey, string token, string payerId)
        {
            //your implementation
        }


       [HttpGet]
        public HttpResponseMessage AbortPayment(Guid invoiceKey, Guid paymentKey, string token, string payerId = null)
        {
            //your implementation
        }
    }

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