I've had a few tires at an online concept store focusing on print-on-demand using Shopify and vendors like Printful and Printify. I loved the ease with which I could splash my designs across a variety of merchandise. I was just having issues with conversions, so it became too costly and I had to close it down. Fortunately, the designs are still waiting for when I want to try again.
Here are some of the product bumpers I created.