I don't think zazzle is a fit for people who are looking to print customer artwork and still make a profit.
Zazzle is more for people who are selling "their own" art/designs/slogans directly to retail (like retail price in a clothing store) customers.
You just need someone who has a
DTG printer who can do "contract printing" for you so you can still make a profit on the order.
Zazzle is more for artists who want to make money on their own artwork by selling direct to the end consumer. Not really a Business to Business type site.