When I uploaded my design I didn't know that semi-transparent colour that fades into clear/transparent background was not supported by this format. The Sticker Mule team very kindly let me know they could not achieve that effect and edited my design by removing such translucent halo in order to show me what could be done. This is peak customer service, so kind and attentive. The stickers look fantastic by the way! I don't regret it. 

Fantastic quality & beautiful finish