Postcards aren’t necessarily just for sending back from your holidays, (though you should always send postcards) - you can use them for mail campaigns, street marketing, appointment reminders, coupons, shopping bag inserts and much more. You can even add your own PPI (speak to Guernsey Post) so that your customers could return them to you at your cost.
The days of bulk emailing are certainly numbered so this is a very good alternative.