Your painting is beautiful. Printing on fabric isn't the same as printing on paper. And there's a learning curve. If you printer doesn't have experience with this, I'd go elsewhere. Spoonflower is a professional company with extensive experience.
Ink seeps into the cellulose fibers. Think of diluting dye in water. Ink attaches to the outside of petroleum fibers, so the same amount of ink is more intense.
Since the Ink seeps into the cellulose fibers, its something like bleeding. Hence it's fussy. This is why there's learning curve to use the right ink and treated fabric. There's several good books on the process.