"Promotional postcards" refers to small cards that are used to advertise or promote a product, service, event, or business. They are typically sent out or handed out to attract attention and inform people about something specific. Full definition
This was the case on my second book, too (although they did spring for some nice promotional postcards). (independentbookworm.com)
In addition to the products mentioned above, Basic Invite offers promotional postcards, welcome kits, printables, save the date announcements, free wedding website services, and much more! (followtheyellowbrickhome.com)
Selected design works from promotional postcards to editorial design by Astrid Stavro Studio, an independent design consultancy. (weandthecolor.com)