As I've pointed out many times, since self-publishing companies sell few books to readers, the companies must make their money by selling services and paraphernalia to writers.
As shown below from the Outskirts Press website, the promotional postcard is a common piece of paraphernalia that writers can buy.
- If you’ve written a book about Shelby Mustangs and can get a mailing list of Mustang fans, your postcards could sell some books. Make sure the card tells where books can be ordered. Consider making a special offer such as free shipping or an autograph or 10% off for orders placed within ten days.