Hello @nebi Thanks for your comment. I dabbled with some of these print designs services last year. What's amazing things you can do with a design!
it's so easy to place the just about anything on a shirt, resize it and place an order.
These ready to print services are really great at helping we as artists turn our work into products.
When I create the art, I found the optimal way to create designs for multiple colors shirts and that's as simple as creating the work as a clean .png file without anything that will show up weird when printed.

Cheers!