I just wouldn't be able to sleep at night selling something to my customer for $300 knowing they can go on Etsy or Amazon or Sticker Mule in your case and get it for for $100 on Amazon/Etsy or for $200 on Sticker Mule
I tell my customers they can buy stickers cheaper on the internet. If they still want me to order them I tell them I will mark them up 40% (less for bigger orders). It doesn't surprise me that many clients prefer I order them despite the markup (I am completely transparent). Clients come to us because of our proven design capability and reliability.
My business model is based on design, project management, and installation, service, and maintenance. I have outsourced almost all printing and fabrication for the last twenty years. Before that I ran a full-service electrical and commercial
sign shop and did most everything in-house. Sales and profits went up and labor and overhead went way down when we started out-sourcing production.