Unfortunately, yes — patent infringement is a strict liability offense. You don't need to know about the patent to be liable. Under 35 U.S.C. § 271(a), "making" a patented invention without authorization infringes, regardless of intent.
However, knowledge affects DAMAGES. If you had no knowledge of the patent:
- No willful infringement → no enhanced/treble damages
- You may be liable for a reasonable royalty on the items made
- Your customer likely owes you indemnification (they brought you the design)
Practical steps: (1) Stop printing the design immediately, (2) notify your customer and demand indemnification, (3) respond to the C&D through an attorney, (4) check if the patent is actually valid and covers what you printed — many C&D letters overstate patent scope.
Going forward: consider adding an indemnification clause to your service agreements requiring customers to warrant they have the right to have the designs manufactured.