I witnessed someone setting up such a business as a side gig. It was pretty straightforward, but they had a background in printing and already had access to printing and cutting machinery. They shut it down in a couple of months. Their takeaway was it is much smaller market one would imagine with plenty of competition. Many players are much more integrated with other kind of custom printing offerings as well (mugs, calendars, t-shirts etc.). Without a unique selling point it was not worth pursuing.