They probably have their own printing equipment if they’re doing it as a business.
I used to have a boyfriend who owned a small print shop (also called a “Service Bureau”) and while his main income/primary customers were glossy color prints for University posters, and medical presentations (his connections were with a University) he also did small custom print jobs (invitations, custom screen printing) for artists/designers selling personalized items, too.

Google “Service Bureaus” to find info.