public void sendPaymentEmail (Map<Id, Id> contactIds) {

        // Get the relevant template Id
        Id templateId = [SELECT Id, Name FROM EmailTemplate WHERE developername = 'PaymentTemplate'].Id;

        // List of emails
        List<Messaging.SingleEmailMessage> paymentEmailsToSend = new List<Messaging.SingleEmailMessage> ();

        // Remove nulls from the contactMap
        for (Id cId : contactIds.keySet())
            if (cId == null && contactIds.get (cId) == null)
                contactIds.remove (cId);


        // List of emails
        List<Messaging.SingleEmailMessage> messagesToSend = new List<Messaging.SingleEmailMessage> ();

        System.debug('#### IDS : ' + contactIds);

        // Create a new email for each contact related to the Registration
        for (Id cId : contactIds.keySet()) {

            Messaging.SingleEmailMessage mailMessage = new Messaging.SingleEmailMessage ();
            mailMessage.setTargetObjectId (cId);
            mailMessage.setSenderDisplayName ('Sedex Admin');
            mailMessage.setTemplateId (templateId);
            mailMessage.setWhatId (contactIds.get (cId));

            messagesToSend.add (mailMessage);

        }

        System.debug ('#### EMAILS TO SEND : ' + messagesToSend);

        Messaging.sendEmail (messagesToSend);

    }