Is there a way I am able to set up the sales of business cards I have printed at Mpix for my customers?

That way they could order them on their own as needed through Zenfolio.

You can definitely set this up as a custom product. Zenfolio will create orders, use your PayPal account to send payment to you directly, and you can then fulfill orders and provide customer service

Check out this page for more information on custom products.